Import Polymer from original dependencies path.

I was trying to use your Custom Element in my project, but it occured it tries to load Polymer dependency from
`bower_components/polymer/polymer.html` which in case you installed `input-currency` via bower is:
`_myproject_/bower_components/input-currency/bower_components/polymer/polymer.html` so it not only requres additional `~_myproject_/bower_components/input-currency/$ bower install`, but also leads to code duplicates, conflicts, etc.

With my change it should use Polymer which was loaded by `bower install input-currency`.