Allow library to coexist with Squeel.

Scope query construction gets in a pickle if Squeel is an included
gem in a project:

Order.in_state(:cancelled)
  #=> ...WHERE "order_transitions"."to_state" IN ("orders"."cancelled")

(Should be: `IN ('cancelled')`.)

Explicitly cast scope arguments to strings to avoid symbols being
interpreted as references to columns on the driving table.
Merge pull request #35 from figwit/fix-squeel

Allow library to coexist with Squeel.