schematool fails on Postgres versions < 9.2

The upgrade script for HIVE-5700 creates an UDF with language 'plpgsql',
which is available by default only for Postgres 9.2+.

For older Postgres versions, the language must be explicitly created,
otherwise schematool fails with the error:

Error: ERROR: language "plpgsql" does not exist
  Hint: Use CREATE LANGUAGE to load the language into the database. (state=42704,code=0)