Testing more Python versions, fix coveralls

Trying to test in more versions of Python. Also fail with reporting coverage to coveralls caused by cd to docs for doctests (probably).

.travis.yml

@@ -1,11 +1,14 @@
 language: python
 python:
 - '3.5'
+- '3.6'
+- 'nigthly'
 install:
 - python setup.py install
 - pip install -r docs/requirements.txt
 - pip install coveralls
 script:
 - python setup.py test
-- cd docs && make doctest
+- cd docs && make doctest && cd ..
+after_success:
 - coveralls