[COOK-2435] Fix foodcritic errors.

FC002: Avoid string interpolation where not required: ./recipes/yum_pgdg_postgresql.rb:50
FC002: Avoid string interpolation where not required: ./recipes/yum_pgdg_postgresql.rb:56
FC007: Ensure recipe dependencies are reflected in cookbook metadata: ./recipes/ppa_pitti_postgresql.rb:1
FC007: Ensure recipe dependencies are reflected in cookbook metadata: ./recipes/ruby.rb:30

I left this one alone:

FC024: Consider adding platform equivalents: ./attributes/default.rb:90

because I'm pretty sure Amazon Linux does things different and that's why there's a separate clause for it.