Extract skggm commit hash to an environment variable

[terrytangyuan]

This makes it easier to upgrade later.

[bellet]

I was confused by the two travis checks but I just realized it must be because @terrytangyuan pushed to a new branch on this repo (not his fork), so I guess this is normal
However I am not sure why there is no test coverage report shown here although a report has been generated, with 2x4 builds (https://codecov.io/github/scikit-learn-contrib/metric-learn/commit/ddca6f3dccbccca902f77da9f00f1154c5b53926)

[terrytangyuan]

Good catch. I’ve fixed the issue with two Travis builds in #242. Not sure about the coverage report though. It looks like a separate issue that needs to be looked into.

[bellet]

Thanks merging