Reduce the load on our CI (#3957)

* changelog

* reduce circleci config

* plus a handy script

* fix regex

scripts-dev/next_github_number.sh

@@ -0,0 +1,9 @@
+#!/bin/bash
+
+set -e
+
+# Fetch the current GitHub issue number, add one to it -- presto! The likely
+# next PR number.
+CURRENT_NUMBER=`curl -s "https://api.github.com/repos/matrix-org/synapse/issues?state=all&per_page=1" | jq -r ".[0].number"`
+CURRENT_NUMBER=$((CURRENT_NUMBER+1))
+echo $CURRENT_NUMBER