Fully support Bundler 2

[chrismo]

• Make sure all specs pass
• use target ruby is wrong in 2.3
• See if you can re-enable 1.9 and 1.10 support in Travis.

[chrismo]

Newer rubies than 2.1 (e.g. 2.3.4) don't put gem bin file in ruby_bin, so this is wrong:

bundler_patch = File.join(tb.ruby_bin, 'bundler-patch')