Print, but don't fail on deprecation notice

A simple change so that deprecation notices are printed, but don't cause test failures. A possible fix for issue #27.
I believe this test is failing (as per your [comment](https://github.com/gds-operations/puppet-syntax/pull/28#issuecomment-66526865)) because the `import` keyword was deprecated in version 3.5 rather than 3.7:
https://tickets.puppetlabs.com/browse/PUP-866
https://docs.puppetlabs.com/puppet/3.5/reference/release_notes.html#deprecations-and-removals

Also, using `else` is unreliable here as it could match Puppet versions prior to 3.7 or 4.0 and above, which means that the context description is misleading.

I was able to make the tests pass by changing the `else` to `elsif`, like this:

```ruby
if Puppet::Util::Package.versioncmp(Puppet.version, '3.7') >= 0 and Puppet::Util::Package.versioncmp(Puppet.version, '4.0') < 0
      context 'on puppet >= 3.7 and < 4.0' do
        it 'should return deprecation notices as warnings' do
          files = fixture_manifests('deprecation_notice.pp')
          output, has_errors = subject.check(files)

          expect(has_errors).to eq(false)
          expect(output.size).to eq(2)
          expect(output[0]).to match(/The use of 'import' is deprecated/)
          expect(output[1]).to match(/Deprecation notice:/)
        end
      end
    elsif Puppet::Util::Package.versioncmp(Puppet.version, '3.5') < 0
      context 'on puppet < 3.5' do
        it 'should not print deprecation notices' do
          files = fixture_manifests('deprecation_notice.pp')
          output, has_errors = subject.check(files)

          expect(output).to eq([])
          expect(has_errors).to eq(false)
        end
      end
    end
```

I ran a [test build](https://travis-ci.org/gds-operations/puppet-syntax/builds/44843485) using this code.