chore(travis): run bower install twice to make sure it does

This is a work-around for Bower/Node.js issue (https://github.com/bower/bower/issues/830). We run `bower install` twice, as the probability of failing twice in a row is very low.

I had to extract `bower` task out of the package, because we need to run `bower install` before building and `grunt bower` can fail, which takes down the whole process and therefore it wouldn't build.