skipif reason must be a string

[lafrech]

On my setup, this fixes a lot of errors while running py.test:

_______________________________________ ERROR at setup of TestPymongo.test_auto_instance ________________________________________
Error evaluating 'skipif' expression
    False
Failed: you need to specify reason=STRING when using booleans as conditions.

[coveralls]

Coverage decreased (-7.03%) to 86.719% when pulling 37f08e9 on Nobatek:skipif_reason_str into 3c5a9cf on Scille:master.

[lafrech]

(I don't understand the coverage decrease.)

The error does not happen when running tox but when running Debian Jessie's py.test.

[lafrech]

I can't explain it, but this change makes the tox report go from

tests/test_data_proxy.py ..............
tests/test_document.py ...................
tests/test_fields.py ....x......
tests/test_i18n.py ...
tests/test_indexes.py .....
tests/test_inheritance.py ....
tests/test_instance.py .........
tests/test_query_mapper.py ..
tests/frameworks/test_mongomock.py s
tests/frameworks/test_motor_asyncio.py ssssssssssssssssssssss
tests/frameworks/test_pymongo.py ..................x..
tests/frameworks/test_txmongo.py ssssssssssssssssssssss

----------- coverage: platform linux, python 3.4.2-final-0 -----------
Name                                 Stmts   Miss  Cover
--------------------------------------------------------
umongo/__init__.py                      13      0   100%
umongo/abstract.py                      72      9    88%
umongo/builder.py                      143      0   100%
umongo/data_objects.py                  54      1    98%
umongo/data_proxy.py                   148     11    93%
umongo/document.py                      89      4    96%
umongo/embedded_document.py             52      0   100%
umongo/exceptions.py                    29      0   100%
umongo/fields.py                       208     17    92%
umongo/frameworks/__init__.py           55     14    75%
umongo/frameworks/mongomock.py           5      3    40%
umongo/frameworks/motor_asyncio.py     199    197     1%
umongo/frameworks/motor_tornado.py       0      0   100%
umongo/frameworks/pymongo.py           187     29    84%
umongo/frameworks/tools.py               9      0   100%
umongo/frameworks/txmongo.py           184    183     1%
umongo/i18n.py                           5      0   100%
umongo/indexes.py                       44      9    80%
umongo/instance.py                      64      1    98%
umongo/query_mapper.py                  24      0   100%
umongo/schema.py                        21      0   100%
umongo/template.py                      22      0   100%
umongo/validate.py                      22      0   100%
--------------------------------------------------------
TOTAL                                 1649    478    71%

to

tests/test_data_proxy.py ..............
tests/test_document.py ...................
tests/test_fields.py ....x......
tests/test_i18n.py ...
tests/test_indexes.py .....
tests/test_inheritance.py ....
tests/test_instance.py .........
tests/test_query_mapper.py ..
tests/frameworks/test_mongomock.py s
tests/frameworks/test_motor_asyncio.py ssssssssssssssssssssss
tests/frameworks/test_pymongo.py sssssssssssssssssssss
tests/frameworks/test_txmongo.py ssssssssssssssssssssss

----------- coverage: platform linux, python 3.4.2-final-0 -----------
Name                                 Stmts   Miss  Cover
--------------------------------------------------------
umongo/__init__.py                      13      0   100%
umongo/abstract.py                      72      9    88%
umongo/builder.py                      143      6    96%
umongo/data_objects.py                  54      1    98%
umongo/data_proxy.py                   148     17    89%
umongo/document.py                      89      4    96%
umongo/embedded_document.py             52      0   100%
umongo/exceptions.py                    29      0   100%
umongo/fields.py                       208     18    91%
umongo/frameworks/__init__.py           55     16    71%
umongo/frameworks/mongomock.py           5      3    40%
umongo/frameworks/motor_asyncio.py     199    197     1%
umongo/frameworks/motor_tornado.py       0      0   100%
umongo/frameworks/pymongo.py           187    144    23%
umongo/frameworks/tools.py               9      7    22%
umongo/frameworks/txmongo.py           184    183     1%
umongo/i18n.py                           5      0   100%
umongo/indexes.py                       44      9    80%
umongo/instance.py                      64      1    98%
umongo/query_mapper.py                  24      0   100%
umongo/schema.py                        21      0   100%
umongo/template.py                      22      0   100%
umongo/validate.py                      22      0   100%
--------------------------------------------------------
TOTAL                                 1649    615    63%

which probably explains the coverage issue.

[lafrech]

This affects other MongoDB frameworks. I can complete the PR if needed.

[touilleMan]

The trouble is only present for pytest<2.8, so I prefer to let it this way.

I've documented the trouble both in doc and in tox.ini