Use quoted action name.

As of Ember 1.4.0-beta.5 using the unquoted version is deprecated. Using a
quoted string is totally backwards compatible so this is a non-breaking
change.

Ember 1.5.0 will use unquoted action names and performed property lookup,
allowing for dynamic actions. We will have to decide how to handle these
dynamic action names at some point
(likely after 1.5.0 ships).

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I also pinned the Sprockets dependency to `>= 2.0.0` which was the first version with `Sprockets.register_engine` (around 2 years ago). Apparently, I had a super old version locally, and could not run the specs.