Update .travis.yml

Update .travis.yml

This commit makes the following changes to the test matrix:

- Runs tests on ruby 2.1.5 instead of 2.0.0
- Runs tests on ruby 1.8.7 with puppet 3.x
- Adds an environment to run on an intermediate 3.x puppet version
  instead of latest (specifically 3.4.x)
- Adds an environment to run with the future parser on latest puppet.
  This would affect the test runs for the validate and spec checks.
- Runs the tests on docker