Fixed gemfile incompatibility with handlebars-source

Bundler is throwing an error on install of gems -> 

"Bundler could not find compatible versions for gem "handlebars-source":
  In Gemfile:
    ember-data-source (>= 0) ruby depends on
      handlebars-source (< 1.0.0.rc4, >= 1.0.0.rc3) ruby

    handlebars-source (1.0.0)"

Fixed by adding version "1.0.0.rc3" to handlebars-source