Remove code and tests for unsupported Python 2.6, 3.3 and 3.4

[hugovk]

c11bb64 says:

Python 3.3 and 3.4 will not be supported officially in next minor version

And 2.6 was dropped in #168.

This removes some 2.6, 3.3 and 3.4 stuff.

Also remove this restriction form .travis.yml to allow me to test this feature branch before creating the PR:

-branches:		
-  only:		
-    - master		
-

I think this is a good thing to keep, but let me know if you'd like it putting back.

Also ignore .cache directory, created from build/test.

[hugovk]

The Docker CI failure also happens on master.

[methane]

I just want don't announce "Python 3.3 and 3.4 are supported."
Don't remove test until it make trouble.

[methane]

Removing code for 2.6 is nice.

[hugovk]

Please see #250.

[hugovk]

Merge conflict resolved.

[hugovk]

By the way, here's the pip installs for msgpack-python from PyPI for the last month:

$ pypinfo --percent --pip msgpack-python pyversion
python_version percent download_count
-------------- ------- --------------
2.7              83.9%        669,038
3.6               6.6%         52,967
3.5               6.2%         49,744
3.4               2.9%         22,903
2.6               0.2%          1,980
3.3               0.1%            484
3.7               0.0%            278
None              0.0%              6
3.2               0.0%              3
3.1               0.0%              1

This shows Python 2.6 and 3.3 have very little usage.

[methane]

OK, simplifying appveyor.yml looks nice.

[methane]

thanks