Add 2.1 to travis build matrix.
Upgrade to RSpec 3.1.
Remove unnecessary mocking.
Fix mutation bug when `serializable_hash` was called by users.
Fully qualify `File` constant to avoid wonky 1.9.2 const resolution issue.

See rspec/rspec-core#1697 for more details.
Deal with slight differences in YAML serialization in 2.1.
Merge pull request #435 from vcr/fix-mutation-bug

Fix mutation bug