Re-enable testing against older iOS SDKs in CI

The more SDKs we work against the merrier! Looks like Travis has re-added
support for testing against the 5.0 and 5.1 iOS SDKs since we had to originally
take them out in this commit:

    8aef6e7ccfbce87b6cf170260ce08e1faf3d7f83
Merge pull request #81 from xoebus/ci-old-sdks

Re-enable testing against older iOS SDKs in CI