Remove the dependency on therubyracer

[ignisf]

therubyracer has long been unmaintained which is causing both security and build
issues to go unfixed. As beef depends on it only to provide a JS runtime for
execjs, we can drop it and require users to install Node.js. execjs
automatically finds the node binary and uses it as a JS runtime.

This fixes #1478, #1045, #1046, #1249, #1374, #1377, #1395, #1396, #1428, #1429
This also fixes #1433, #1502, #1504, #1540, #1545, #1547

[ignisf]

I am not able to run all tests due to the following error:

Errno::ENOENT: No such file or directory @ rb_sysopen - tmp/rk_beef_conf.yaml

[bcoles]

Yeah that test is poorly designed

[bcoles]

This should probably fix it:

mkdir tmp
cp config.yaml tmp/rk_beef_conf.yaml

Ideally the test should be fixed to not suck.

[bcoles]

Thanks

[blshkv]

guys in the bug above suggested to have a look at mini_racer. FYI