Remove old Build class when using Build.pm

If two packages with Build.pm files were installed during the same panda
run (example: "panda install MIME::Base64 Auth::PAM::Simple"), the
second and subsequent packages would all fail to build, as the Build
class already existed when the Build.pm file attempted to create it.
Merge pull request #75 from retupmoca/master

Remove old Build class when using Build.pm