Travis is not passing on rubies 1.9 and 2.0

[abinoam]

See:

• Previous .travis.yml: https://travis-ci.org/apotonick/disposable/builds/186775154
• Updated .travis.yml: https://travis-ci.org/abinoam/disposable/builds/196008704

It's probably related to dry-rb dependency. Dry-rb has dropped support for old rubies.

Related to: https://github.com/trailblazer/representable/issues/208

[fran-worley]

Travis has been updated via #72