Checks now if a newer version of Ember and Handlebars is available (with fallback files).

I was facing a problem using ember-precompile : the precompiled templates were not compatible with last version of ember I was using in my project. 
I added the functionnality to check if newer versions of Ember and Handlebars are available before precompiling template. If not ember-precompile act as usual, with (updated) fallback files.

Thanks a lot!