travis: Trim down the matrix, simplify versions.

Throw away anything relating to Puppet 2.6 and Puppet 3 versions prior
to 3.4. The latter is kept to ensure compatibility with the previous
release of PE, 3.2.

By using the pessimistic version operator, `~>` we can simply specify
`2.7.0` and be sure that we'll get the latest version in the 2.7.X
series.

Ruby 1.9.2 was never supported by Puppet Labs, no need to test for it
and Puppet 2.7 was never supported on Ruby 1.9.3, as per:
https://docs.puppetlabs.com/guides/platforms.html