Update Travis Rubies/actual allowed failures

The syntax for allowed failures requires the block to be nested in a "matrix"
key. This also cleans up the syntax used for specifying the test command since
there's only one.
Use mspec from RubyGems.

This fixes that various Bundler Git errors on Travis. This commit also removes
the dependency on ffi since that's not used anywhere.
Merge pull request #307 from rubyspec/bundle-pain

Ditch Git dependencies in favour of just mspec