Respect MACHINE_DRIVER environment variable for docker-machine.
Respect MACHINE_DRIVER environment variable for docker-machine.

This solves issue #15.

larskanis/rake-compiler-dock,larskanis/rake-compiler-dock,rake-compiler/rake-compiler-dock,rake-compiler/rake-compiler-dock,rake-compiler/rake-compiler-dock,rake-compiler/rake-compiler-dock,larskanis/rake-compiler-dock,rake-compiler/rake-compiler-dock