However, the migration_modules directory is present in the 1.5 release at gitub. Thus, it seems that this is simply a mistake of not including the migration_modules directory in the PyPi package. Could simply be resolved by including it in a 1.5.1 release to PyPi.
The package located at https://pypi.python.org/pypi/cmsplugin-simple-markdown is missing the migration_modules directory. This causes migrations to fail with InvalidBasesError.
However, the migration_modules directory is present in the 1.5 release at gitub. Thus, it seems that this is simply a mistake of not including the migration_modules directory in the PyPi package. Could simply be resolved by including it in a 1.5.1 release to PyPi.