Updated Cython version to >=0.19 in docs

[aniketk21]

@astrofrog Updated Cython version number to >=0.19 in docs/install.rst #4705

[aniketk21]

@astrofrog Travis CI tests failed with the following error. What does it mean?
Traceback (most recent call last):
File "/home/travis/miniconda/envs/test/bin/pip", line 6, in <module> sys.exit(pip.main())
File "/home/travis/miniconda/envs/test/lib/python2.7/site-packages/pip/__init__.py", line 215, in main locale.setlocale(locale.LC_ALL, '')
File "/home/travis/miniconda/envs/test/lib/python2.7/locale.py", line 579, in setlocale return _setlocale(category, locale)
locale.Error: unsupported locale setting

[bmorris3]

It looks to me like the travis job needs to be restarted.

[pllim]

Looks more like this needs to be rebased so it picks up the fix in #4717.

[eteq]

It'll be fine when merged, though, so I'll go ahead and merge it. Thanks @aniketk21!