Add db2 and informix as VALID Rails relational database types

Hi,

This PR adds DB2 and informix to the list of valid Rails db types. This PR also provides mapping of db2 and informix database adapter to ibmdb [the adapter name to be specified for using the ibm_db ruby gem to connect to db2 or informix database, https://rubygems.org/gems/ibm_db ].

I have added unit test too to check that db2 and informix is recognised as valid DB type and the mapping to ibmdb is working fine

Thanks

Praveen