.travis.yml workaround allowing Travis-CI build to run.

Un-sets `GEM_PATH` in `before_install` phase of Travis build configuration.  This does not affect builds outside of Travis.

The `.travis.yml` customization can be retired once uPortal is pulling in a fixed version of the SASS Maven compile plugin, presumably the forthcoming version `1.1.2`.

The `README.md` image will of course only make sense once this pull is merged to `master` and Travis-CI continuous integration is turned back on.

See [https://travis-ci.org/apetro/uPortal/builds/18151771](successful build of apetro/master containing this .travis.yml change) for demonstration that the proposed `.travis.yml` results in a working Travis-CI build.