Resolve rubygems dependency issue with ember-source

Because ember-source has a complex handlebars-source dependency the dependency resolver on rubygems.org is having a hardtime reconciling this with the handlebars-source dependency version in ember-dev. It is true they should both be OK but in reality they are causing a conflict.

This will set the version of handlebars-source to match the dependency version in ember-source to resolve this issue.