Update .gemspec to depend on schema_plus_core 2.0
Update .gemspec to depend on schema_plus_core 2.0

Signed-off-by: Alex Coles <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@alexbcoles.com>

SchemaPlus/schema_plus_pg_indexes,patientslikeme/schema_plus_pg_indexes