Made the main library file loadable.

high_voltage.rb uses mattr_accessor methods, which are provided by
active_support. Normally these methods are provided by having Rails ran
beforehand, but this meant that high_voltage couldn't be loaded alone.

Added necessary require to address this.
Merge pull request #72 from thoughtbot/mm-mattr_accessor-requiring

lib/high_voltage.rb should be able to be required.