Remove old hack to support pre-0.7.5 Alembic versions

Thanks to Jessica Tallon for pointing out this TODO and its associated
hack could be removed.

mediagoblin/db/migration_tools.py

@@ -46,9 +46,6 @@ class AlembicMigrationManager(object):
         alembic_cfg_path = os.path.join(root_dir, 'alembic.ini')
         self.alembic_cfg = Config(alembic_cfg_path)
         
-        # TODO: After 0.7.5 alembic has Config.attributes already made, once
-        #       we're able to update, please remove this hack!
-        self.alembic_cfg.attributes = {}
         self.alembic_cfg.attributes["session"] = self.session
         self.alembic_cfg.set_main_option("qlalchemy.url", str(self.engine.url))
 

setup.py

@@ -49,7 +49,7 @@ if PY2:
 
 install_requires = [
     'waitress',
-    'alembic',
+    'alembic>=0.7.5',
     'python-dateutil',
     'wtforms',
     'py-bcrypt',