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