Make Ruby Racer Optional

[catmando]

Instead of requiring ruby racer we should:

Check if it is present, if so use it for prerendering (if prerendering config is on)
if not present and prerendering config option is on, then print an warning, that prerendering is being turned off.
if not present and prerendering config is off, then no action is needed.

Note: prererending config is on by default.

[sfcgeorge]

Can't Node be used if present? I feel like that's what other tools do and therubyracer isn't recommended.

[janbiedermann]

Node cant currently be used, as in prerendering hyperloop calls back into ruby rails context, which node currently cant do. But therubyracer is upgraded to mini_racer.