Fixes a long-standing bug in import under Python 3.3 and later.

Our MetaImporter was being inserted at the end of sys.meta_path.
For Python prior to 3.3, this was fine since sys.meta_path
was empty by default. As of the completion of PEP 302 in Py3.3 and
later, there are several importers registered by default. One of
these was trying (and failing) to import simple Hy modules,
resulting in a failure to import anything inside __init__.hy.

This change simply inserts the Hy-specific importer at the front
of the list.

This was noted in issue #620 (great catch @algernon)
Merge branch 'master' into pr/706