Run `bower install` before running tests on Travis CI

Since 7ad66fbaefee5491eed192773802d6ff7fb9bb07, you need to run `bower install` on Travis CI.

As a result:
- `bower` has been added to the `devDependency` list in `package.json` ;
- `bower install` has been added as a `before_script` in `travis.yml`.