(maint) refactor to remove File.read stubs

Prior to this commit, the tests for SettingsReader.filename_to_hash were
stubbing File.read. This commit does a minor refactor of
SettingsReader.filename_to_hash and associated tests to allow
expectations and stubbing to be done on our local methods rather than on
File.read. We can safely assume that File.read works the way it ought.
(maint) bump version of Mocha, pg, & activerecord

Prior to this commit, the tests were experiencing strange failures due
to an errant $SAFE being set. The root cause was never determined, but
the problem goes away with the newer versions of the gems included in
this commit.