Grunt: run more JS validation tests and `checktextdomain` during the `travis` task.

While committers ought to run `grunt build-commit` prior to committing, this is intended to act as a safety net should we forget and overlook something. Hopefully, the failures will bubble up to Travis-CI, and we'll be notified that way.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9228 cdf35c40-ae34-48e0-9cc9-0c9da1808c22