Update vagrant.gemspec

Resolves this bundler issue:

Bundler could not find compatible versions for gem "net-ssh":
  In Gemfile:
    vagrant (>= 1.3.5) ruby depends on
      net-ssh (~> 2.6.6) ruby

    test-kitchen (>= 1.0.0.rc.2) ruby depends on
      net-ssh (2.7.0)