Update migrate and create_migration to work right with lein 2

Updated create_migration.clj and migrate.clj to work correctly with lein
2. Fixes #16.

The updates are explained here at https://groups.google.com/forum/?fromgroups=#!topic/leiningen/dUeYm0NsF08 and in the leiningen readme https://github.com/technomancy/leiningen/blob/master/doc/PLUGINS.md#code-evaluation.