Beef up the box
Beef up the box

* 2 GB RAM by default
* 2 CPUs by default

Both configurable via ENV vars.

Implementation lifted from rails-dev-box: https://github.com/rails/rails-dev-box/blob/7cb1f29fcb21bd54460385c890097e38f28d52fb/Vagrantfile#L11-L14
tjschuck/rake-compiler-dev-box,tjschuck/rake-compiler-dev-box,tjschuck/rake-compiler-dev-box,tjschuck/rake-compiler-dev-box