Test multiple Python versions with Travis

Based on <http://conda.pydata.org/docs/travis.html>. Test with both
Anaconda and native Ubuntu Python interpreters.
Don't use C99 designated initializers

It causes some build failures in Travis CI.
Wrap travis_retry around pip commands

Sometimes there are network glitches which cause `pip` to fail with the
message `Connection reset by peer`. If we wrap `pip` commands with
`travis_retry`, these commands will be tried up to three times before
causing a build failure.

See <http://docs.travis-ci.com/user/build-timeouts/>.
Skip Python 3.4 if not using Anaconda

The Ubuntu package used by Travis seems to use different default
compiler flags.
Disable internal cfitsio builds for Python 2.6 and 3.3

These configurations are almost redundant, and they are some of the
slower configurations.
Merge pull request #188 from lpsinger/multi_python_travis

Test multiple Python versions with Travis: 2.6, 2.7, 3.3, 3.4