Revert the change to the recaptcha import test.

For some mysterious reason, recaptcha doesn't always work with
imp.find_module(). It works fine in my virtualenv, but not in Christian's,
despite everything looking very similar.

This changes our test back to using 'import', and crucially adds a comment
explaining why we use that over imp.find_module().

Reviewed at https://reviews.reviewboard.org/r/5776/