Change version function from IMMUTABLE to STABLE

These functions' results will change when the extension
is updated.

src/pg/sql/01_version.sql

@@ -2,11 +2,11 @@
 CREATE OR REPLACE FUNCTION cdb_crankshaft_version()
 RETURNS text AS $$
   SELECT '@@VERSION@@'::text;
-$$ language 'sql' IMMUTABLE STRICT;
+$$ language 'sql' STABLE STRICT;
 
 -- Internal identifier of the installed extension instence
 -- e.g. 'dev' for current development version
 CREATE OR REPLACE FUNCTION _cdb_crankshaft_internal_version()
 RETURNS text AS $$
   SELECT installed_version FROM pg_available_extensions where name='crankshaft' and pg_available_extensions IS NOT NULL;
-$$ language 'sql' IMMUTABLE STRICT;
+$$ language 'sql' STABLE STRICT;