Security: Remove all traces of therubyracer from Rails

[SamSaffron]

therubyracer is a relic, it depends on an ancient version of v8 that has multiple known security vulnerabilities

https://github.com/cowboyd/therubyracer/blob/master/therubyracer.gemspec#L20

In particular this is the equivalent of shipping a dependency to Chrome version 31.

Chrome 31 was released 2013-11-12

All places that talk about therubyracer or include it in templates should be replaced with mini_racer that depends on the most recent version of v8.

[SamSaffron]

Only 4 spots, so changing this should be fairly easy:

https://github.com/rails/rails/search?utf8=%E2%9C%93&q=therubyracer&type=

[maclover7]

@SamSaffron Can you open a PR, and then we can continue the discussion there? Thanks!