Update Gemfile

Bump default Vagrant test version to 1.6.3 and make it configurable via the
`TEST_VAGRANT_VERSION` environment variable. Set required Ruby version to
'2.0.0' for compatibility with Vagrant 1.4 and newer.
Add spec test skeleton

Add dependency on RSpec 2.14.x and various sundry files.
Add spec tests for global config

Three sets of tests that assert the default config passes validation and
exercise the validation routines for `version` and `download_root`.
Update documentation for global config

Add docstrings to config attributes and tag with `@since` version numbers.
Specify markdown as the YARD markup format