Change version to 1.3.1.

Change version to 1.3.1.

1.3.0 has a crash bug in Describe message of extended protocol
path. If server receives the message after invalidation of
corresponding query cache, pg_hint_plan crashes.

On the way fixing it, this version took in the following commits of
core and pg_stat_statements.

8b6294c7a5 Change more places to be less trusting of RestrictInfo.is_pushed_down.
83f2061dd0 Teach contrib/pg_stat_statements to handle multi-statement commands better.
0bb51aa967 Improve parsetree representation of special functions such as CURRENT_DATE.