chore(Rakefile): paralelize closure compilation
chore(Rakefile): paralelize closure compilation

this speeds up the build by paralelizing closure compilation (the slowest
piece of the build process)

before:

real  0m14.372s
user  0m31.649s
sys   0m1.006s

after:

real  0m8.191s
user  0m40.473s
sys   0m1.378s

ggershoni/angular.js,ggershoni/angular.js,ggershoni/angular.js,ggershoni/angular.js