Remove "require 'bundler/setup'"

This causes problems for applications that use puppet_forge as
libraries, for example librarian-puppet, as seen in the second error
message on
https://github.com/rodjek/librarian-puppet/issues/249

If librarian-puppet is installed via gem, but puppet_forge is not
included in the Gemfile that belongs to the current working directory,
the following error will occur:

.../gems/puppet_forge-1.0.2/lib/puppet_forge.rb:7:in `require':
        cannot load such file -- puppet_forge/version (LoadError)

This is caused by requiring 'bundler/setup', which limits $LOAD_PATH
to gems included in the current bundle, thus removing puppet_forge's
own gem path from $LOAD_PATH.