Travis CI improvements

[mnlevy1981]

Can py_requirements.txt point pip to the version of sphinx_rtd_theme we are using? Talking with @matt-long, there are several possibilities based on pip's ability to point to github:

1. See if the rtfd/sphinx_rtd_theme branch created for #438 is the same as what we got from Unidata... if so, use that; if not, modify conf.py to be compatible with this branch
2. See if Unidata is interested in hosting their version of sphinx_rtd_theme in github rather than passing a .whl file around
3. Fork rtfd/sphinx_rtd_theme into marbl-ecosys organization, and either (a) use the version-dropdown branch mentioned in first option or (b) create a branch with the Unidata version of the package, then py_requirements.txt can point to our github organization.

I'm leaning towards option 1 or 3a -- it seems likely that if rtfd does eventually bring in support for a version dropdown, it will be configured in the same way as that PR... so it should be straight-forward to switch from the old branch to a new release containing the feature.

[mnlevy1981]

It turns out the version-dropdown branch used in the pull request linked above is identical to the code I got from the Unidata group. PR #294 uses option 3a described in the first comment, which was easier to implement than option 1 because the rtfd/sphinx_rtd_theme master branch has some weird properties in .gitattributes and that led to pip compaining when I tried to install the branch on my laptop:

$ pip install git+https://github.com/rtfd/sphinx_rtd_theme.git@version-dropdown
Collecting git+https://github.com/rtfd/sphinx_rtd_theme.git@version-dropdown
  Cloning https://github.com/rtfd/sphinx_rtd_theme.git (to revision version-dropdown) to /private/var/folders/_l/75__d3ks5b54x_gklwjy2gc1_cwn99/T/pip-req-build-OyQT1b
error: Your local changes to the following files would be overwritten by checkout:
	docs/make.bat
Please commit your changes or stash them before you switch branches.
Aborting
Command "git checkout -q b46bba51649fd15b90e6938b57a1e96f7063c81f" failed with error code 1 in /private/var/folders/_l/75__d3ks5b54x_gklwjy2gc1_cwn99/T/pip-req-build-OyQT1b

There's an explicit text eol=lf that results in docs/make.bat being modified immediately after cloning, and then pip can't change from master to version-dropdown due to the changes in that file. So I forked rtfd/sphinx_rtd_theme to the marbl-ecosys org and made version-dropdown the default branch.

[mnlevy1981]

closed via #294