restrict rspec-core gem to < 3

To work around rspec 3 being incompatabile with rspec-puppet 1.0.1 (currently,
the latest release)

    RSpec::Puppet::FunctionMatchers::Run implements a legacy RSpec matcher
    protocol. For the current protocol you should expose the failure messages
    via the `failure_message` and `failure_message_when_negated` methods.
    (Used from /home/jhoblitt/github/puppet-nsstools/spec/functions/nsstools_add_cert_spec.rb:21:in `block (2 levels) in <top (required)>')

Gemfile

@@ -14,3 +14,5 @@ gem 'puppet-syntax',                  :require => false
 #   https://github.com/rodjek/rspec-puppet/pull/155
 #   https://github.com/rodjek/rspec-puppet/commit/03e94422fb9bbdd950d5a0bec6ead5d76e06616b
 gem 'rspec-puppet', '1.0.1', :git => 'https://github.com/rodjek/rspec-puppet.git', :ref => '03e94422fb9bbdd950d5a0bec6ead5d76e06616b', :require => false
+# rspec 3 breaks rspec-puppet 1.0.1
+gem 'rspec-core', '~> 2.0',           :require => false