* test/rubygems/test_gem_commands_setup_command.rb: @ui uses StringIO
  as its streams, and Encoding.default_external does not effect to
  stringIOs already exist.  so, we need to set external_encoding of
  @ui.outs directly.  this problem (test failure) does not appear in
  the environments default_external is us-ascii or utf-8.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47624 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
Enable Multi-OS feature

Also turn off notifications and web hooks
Allow OS X jobs to fail
Allow 'diff' to fail and move on
Limit jobs for OS X

Seems to trigger build failures which manifest as
"Resource temporarily unavailable"
Disable multi-threading in make on the Mac

Race condition appears to trigger build failures

See, for example,
https://travis-ci.org/BanzaiMan/ruby/jobs/35615315
https://travis-ci.org/BanzaiMan/ruby/jobs/35615317
Use Homebrew's OpenSSL on OS X