rubygems 2.4.1 compatibility

Use `Gem::Specification.find_all_by_name('metasploit-yard').first`
instead of `Gem::Specificiation.find_by_name('metasploit-yard')` as
rubygems 2.4.1 fixed `Gem::Specification.find_by_name` so it doesn't
return pre-release gems like it did in rubygems 2.2.2.