Travis CI: Fix build script

We need to pass PG_CONFIG to "sudo make install" because sudo clears
the PATH and so it might run the install step against another version.

.travis.yml

@@ -22,5 +22,5 @@ script: |
   sudo apt-get install postgresql-$PGVERSION postgresql-server-dev-$PGVERSION
   sudo pg_createcluster --start $PGVERSION test -p 55435 -- -A trust
   make all
-  sudo make install
+  sudo make install PG_CONFIG=/usr/lib/postgresql/$PGVERSION/bin/pg_config
   PGPORT=55435 make installcheck PGUSER=postgres