Removed Jruby specific fix and disabled global gemset again by default

- removed jruby specific workaround (this was fixed in ruby-build sstephenson/ruby-build@76b6328). Code was also tested against a manually installed Jruby 1.7.11 with and without `globals` in gemset list

- removed the default inclusion of the 'global' gemset, rbenv-gemset is a tool for clearly defined gemsets, so no gemset should be "enforced". If the user really wants the global gemset included, it's easy to do so by including 'global' gemset in either the .rbenv-gemsets file or in the RBENV_GEMSETS env variable (fixes jf/rbenv-gemset#59)