Add Support for Sprockets 3.0

[alexkravets]

1. Add support for sprockets 3.0, thanks to @hugocorbucci, @stevenchanin
2. Keep support for sprockets < 3.0, thanks to @rafamanzo

[jfirebaugh]

Thanks @alexkravets, @hugocorbucci, @stevenchanin, @rafamanzo.

This is close but not quite there. In 7a98b4b74697de75ea5e384c4c72e36196487823 I checked that master is still passing tests with sprockets-rails 2. In e27938b7f8b55145d2242c027525bc4881de0ba0 I adjusted the test matrix so that the following combinations are tested:

• Rails 4.2, sprockets 3, sprockets-rails 3 (failing before this PR, passing after, :+1:)
• Rails 4.1, sprockets 3, sprockets-rails 3 (failing before this PR, passing after, :+1:)
• Rails 4.2, sprockets 2, sprockets-rails 2 (passing before and after this PR, :+1:)
• Rails 4.2, sprockets 3, sprockets-rails 2 (passing before this PR, failing after, :dizzy_face:)

You can see the failure on the last one here.

One thing to note is that konacha already had support for sprockets 3. The upgrade in question is sprockets-rails. So I believe some of the version checks in the PR may need to change from Sprockets::VERSION to Sprockets::Rails::VERSION.

[alexkravets]

Still fails, i'll figure this out a bit later.

[jfirebaugh]

Merged in 71b47cc2007d991be5cfcd31d719fd14a15847cb and 857dd37721158b7287b1b0d06a638b03e27907dc and will be released in 4.0 shortly. Thanks everyone!