Conditional gem source

If USE_OFFICIAL_GEM_SOURCE is set then rubygems.org is used,
otherwise taobao.org mirror is used.
Try bundle with USE_OFFICIAL_GEM_SOURCE for travis
Set USE_OFFICIAL_GEM_SOURCE environment variable for travis
Update README about USE_OFFICIAL_GEM_SOURCE [ci skip]
Merge pull request #283 from ruby-china/gem-source

Conditional gem source