Provide a translation for 'false'

Fixes error seen in this build:
https://travis-ci.org/samvera/sufia/jobs/272172488
No idea why this started failure just now; relevant code was introduced
2 years ago:
https://github.com/samvera/curation_concerns/blob/56fc90481bb72da2e9c3beb4ebe0f7aa83d5aa8f/app/renderers/curation_concerns/configured_microdata.rb#L6

config/locales/sufia.en.yml

@@ -1,4 +1,5 @@
 en:
+  false: false
   sufia:
     admin_sets:
       index: