Drop Rails < 4.2 to support current Rails

[tricknotes]

Some tests on Sprockets 4 beta are failed due to Sprockets' internal changes.

• Add support for Sprockets 4 #532
• Allow ember-es6_template 0.5.x #511

It is hard to keep supporting wide range of Rails versions.
Additionally, now Rails < 4.2 is officially unmaintained.

• http://guides.rubyonrails.org/maintenance_policy.html

I think it is the time to drop legacy Rails to support current Rails.