Don't attempt to show satisfaction survey when not present.

This has been causing ramdom (1 in 50) failures in our javascript
integration tests because they use a minimal layout that doesn't include
the satisfaction survey markup (see
https://github.com/alphagov/slimmer/blob/master/lib/slimmer/test_template.rb).
Merge pull request #309 from alphagov/fix_satisfaction_survey

Don't attempt to show satisfaction survey when not present.