Add osx to travis

[aaraney]

Added travis support for testing on mac. Related to #30

This will run the prior three python version tests on linux plus the same tests now on Mac.

I also moved much of what had been in install into before_install just for cleanliness. I ran into some issues with the Mac virtual machines travis was using when using BSD sed (that is the sed that is pre-installed on Mac) to replace the python version number in environment_ci.yml. So, to overcome that, I just installed GNU sed using homebrew.

[codecov[bot]]

Codecov Report

Merging #31 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #31   +/-   ##
=======================================
  Coverage   72.46%   72.46%           
=======================================
  Files           5        5           
  Lines        1086     1086           
=======================================
  Hits          787      787           
  Misses        299      299           

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[cheginit]

That's great, thanks!