Configure manifest directory in spec_helper

This prevents the "No file(s) found for import of ... site.pp" error.
Pin rspec-puppet at 1.0.x

Good news: version 1.0 of Rspec puppet was released in December of 2013 which
contains fixes to bugs that prevented the test suite of this moudle from
running under version 0.1.6.

Bad news: Breaking changes have already been made. In the near future,
`subject` will have to be swapped for `catalogue` when testing Template output.

Good news: Breaking changes will be in version 2.0.0. For now, we can pin to
version 1.0.x through Rubygems and stop using the git repo HEAD.
Update test matrix

Three changes:

  - Add Ruby 2.0.0 to the matrix.

  - Test against the latest 2.7.x and 3.4.x.

  - Use `:require => false` for all Gemfile dependencies.
Don't test Puppet 2.7.x under Ruby 1.9.3 or 2.0.0

Puppet 2.7.x does not support these versions. 1.9.3 might work by coincidence,
but 2.0.0 definitely fails.