Update testing dependency and use jc.ErrorIsNil

Replaced all occurrences of
    c.Assert(err, gc.IsNil)
with
    c.Assert(err, jc.ErrorIsNil)

Also replaced `c.Check` variants.

While I was making such a massive change, I also changed:
     gc.Equals, true)
to
     jc.IsTrue)

and
    gc.Equals, false)
to
    jc.IsFalse)