`be_true` and `be_false` are deprecated

When working on https://github.com/puppetlabs/puppetlabs-inifile/pull/115, I was getting a load of noisy deprecation warnings. I fixed those up. In order to make these specs pass, I changed the predicate to return a boolean.

> Methods that don't return a boolean, shouldn't end in a question mark.
-- https://github.com/bbatsov/ruby-style-guide#naming