Fix rspec 3.x syntax

- Convert 'should' keyword to 'is_expected.to'
- The old ':should' syntax in rspec 3.x is deprecated in favor of
  ':expect' syntax.

Change-Id: I80296c6016c02f5991401a4dd1dfd54360cceb7c
Signed-off-by: Gael Chamoulaud <gchamoul@redhat.com>

spec/functions/get_ext_net_name_spec.rb

@@ -4,7 +4,7 @@ describe 'get_ext_net_name' do
   let(:scope) { PuppetlabsSpec::PuppetInternals.scope }
 
   it 'should exist' do
-    Puppet::Parser::Functions.function('get_ext_net_name').should == 'function_get_ext_net_name'
+    expect(Puppet::Parser::Functions.function('get_ext_net_name')).to eq('function_get_ext_net_name')
   end
 
   it 'should return the network name that has router_ext enabled' do