Use rubygems only if they exist

[kagminjeong]

I'm trying to test integrity which uses dm-sweatshop, and receive the following error:

/usr/local/lib/ruby/vendor_ruby/1.8/dm-core.rb:26: warning: already initialized constant Mash
/usr/local/lib/ruby/vendor_ruby/1.8/dm-sweatshop/unique.rb:66: undefined method `gem' for DataMapper::Sweatshop::UniqueWorker:Class (NoMethodError)
    from /usr/local/lib/ruby/vendor_ruby/1.8/dm-sweatshop.rb:6:in `require'
    from /usr/local/lib/ruby/vendor_ruby/1.8/dm-sweatshop.rb:6
    from ./test/helper.rb:4:in `require'
    from ./test/helper.rb:4
    from ./test/unit/build_test.rb:1:in `require'
    from ./test/unit/build_test.rb:1
    from /usr/local/lib/ruby/vendor_ruby/1.8/rake/rake_test_loader.rb:5:in `load'
    from /usr/local/lib/ruby/vendor_ruby/1.8/rake/rake_test_loader.rb:5
    from /usr/local/lib/ruby/vendor_ruby/1.8/rake/rake_test_loader.rb:5:in `each'
    from /usr/local/lib/ruby/vendor_ruby/1.8/rake/rake_test_loader.rb:5
rake aborted!

gem should only be called if it exists.