Template handler deprecation warnings using JS Notifier

The airbrake_javascript_notifier throws a deprecation warning in rails 3.

`DEPRECATION WARNING: Passing a template handler in the template name is deprecated. You can simply remove the handler name or pass render :handlers => [:erb] instead. (called from block in vanity_js at /Users/Hiroki/.rvm/gems/ruby-1.9.3-p0@grapi/bundler/gems/vanity-5ab986ba8026/lib/vanity/frameworks/rails.rb:220)`

This is a pull-request for https://github.com/airbrake/airbrake/issues/205