Ensure "alembic current" won't unnecessarily mutate the database

Ensure "alembic current" won't unnecessarily mutate the database

The ``alembic current`` command no longer creates an ``alembic_version``
table in the database if one does not exist already, returning no version
as the current version. This allows checking for migrations in parallel
without introducing race conditions.  Pull request courtesy Nikolay
Edigaryev.

Fixes: #694
Closes: #695
Pull-request: https://github.com/sqlalchemy/alembic/pull/695
Pull-request-sha: fd3e3b8faf7a41dd4c35daca6c7d224e983ab496

Change-Id: I500ab9ec1fe74b5e20e6aecfe598bce7e9cdef96