PE-1504: drop ruby 1.8.7 and add ruby 2.0.0 to the projects's travis-ci matrix

This patch updates the project's travis-ci matrix as stated in the title
to make the tests more useful. (As the tests didn't run with ruby 1.8.7
because the prerequisite nokogiri gem requires ruby version >= 1.9.2 and
ruby 2.0.0 was not covered at all.)
PE-838: default to installing PE during bootstrap

This patch changes the default value of the --install-script
option of the:
  puppet node_aws bootstrap
command to 'puppet-enterprise'.
While at it the option verification is improved so that missing
options are detected early rather than halfway through the
bootstrap process.
Further the vocabulary of options' help texts and related error
messages is unified.
PE-838: update tests for the change in the install script default

This patch updates tests for the change in the default value of the
--install-script option. While at it, some expectations are made more
specific and the test code is modernized.
PE-838: update rspec syntax to kill deprecation warnings

This changes rspec's
  expect { }.should
constructs, to:
  expect { }.to
to avoid deprecation warnings.
Merge pull request #117 from mruzicka/PE-838_default_to_PE_install

PE-838: default to installing PE during bootstrap
Merge pull request #119 from mruzicka/PE-1504_travis-ci_matrix_update

PE-1504: drop ruby 1.8.7 and add ruby 2.0.0 to the projects's travis-ci matrix