prevents #'contains? being called on strings and booleans for 1.5

Clojure 1.5's #'contains? is stricter about how it is called (see
[CLJ-932](http://dev.clojure.org/jira/browse/CLJ-932)).  Stencil was passing all sorts of objects to #'contains? (via
  quoin), so this guard prevents that from happening.