Bump libgit2 to 31e752b6546537bbeee89b6d2f3027cf0eff9a53
Add install directions for cmake on OS X

rugged 0.21.0 does not install correctly without cmake. Thanks.
Fix script/cibuild
Fix test suite
Bump Ruby version for CI
Respond to the ssh username query

The ssh backend wants to know the username in order to connect to the
server and figure out what authentication mechanisms it supports, so we
need to catch that before we check whether the user's provided
authentication mechanism is allowed.
Explicit returns
Remove "without username" test
travis: explicitly allow user/pass logins

This makes sure we know what to expect in the tests.
GitHub only allows publickey authentication over SSH

We used to test for plaintext as well because libgit2 would pretend to
know and just say everything is supported.
Merge pull request #414 from libgit2/vmg/fix-build

Fix the build with libgit2/master
Add Repository#attributes
Tests
Rugged::Repository::Attributes
Fix option parsing
Merge pull request #412 from dankohn/patch-1

Add install directions for cmake on OS X
Merge pull request #411 from libgit2/vmg/attributes

Add support for Git Attributes
0.21.1b0
Factor File.join in test sandbox_init.
Gitignore rdoc/.