Add underscore to Bower dependencies

Since 18aeb777ebe79ee6db5dfdaf0981ca734c329180, `angular-google-maps` requires `underscore`. However, this library has not been added to the dependency list in `bower.json`. As a result, any AngularJS application that uses `angular-google-maps` without loading `underscore` first will crash.

Please note that I only added the missing entry in `bower.json`.

Maybe it would be a good thing to run `bower install` (manually or via https://github.com/stephenplusplus/grunt-bower-install) before running tests and to use these librairies instead of those versioned.