Update Travis build process to generate resolution-independent CSS files.

### Issue
Now that have modified our LESS process for generating CSS files, the CSS files generated from Travis no longer match the resolution-independent CSS files.

### Fix
We add the use of the `ri` flag to the Travis build script, and modify the Travis build configuration so that it utilizes the base branch of any pull request for the cloning of `enyo`, which should contain our updated LESS compiler script (i.e. pull requests made against `2.5-gordon`).

### Notes
Alternatively, we could hardcode the cloning of enyo's `2.5-gordon` branch in the Travis configuration file for a simpler fix.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>