Add therubyracer gem

For some reason, when this app is being deployed in nginx+passenger (in a Docker container?), despite NodeJS being installed and everything working using 'rails server'... I get the "Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes." message back from Passenger when accessing http://hostname:80

Signed-off-by: Ippoippo <ippoippo-software@ippoippo-eco.com>