add license information to gemspec
add license information to gemspec

This way you can get this info when using rubygems.org API

vhl/fakefs,1234-/fakefs,pivotal-cf-experimental/fakefs,bf4/fakefs,defunkt/fakefs,marxarelli/fakefs,bquorning/fakefs,onehub/fakefs,eventualbuddha/fakefs,anthonyryan1/fakefs,fakefs/fakefs,deployable/fakefs,dekz/fakefs