Move require into method to avoid circular require

There is a nasty circular dependency here:

formula.rb
  requirement.rb
    extend/ENV.rb
      extend/ENV/shared.rb
        formula.rb

Probably the information that the build environment needs from formula
should be passed in at runtime, but that seems hard and I really just
want to be able to run the tests with warnings turned on. :/
Add missing require
gnupg2: fix dependency

Both the bottle and source builds link against "regular" pth, which is
pulled in as a transitive dependency by gpg-agent. The npth changes
appear to be on the 2.1 branch.
Move constants so we don't have to load all of ENV to get them
git 2.0.1
git: update 2.0.1 bottle.