Use wget rather than curl

the docker image doesn't have wget

.circleci/merge_base_branch.sh

@@ -16,7 +16,7 @@ then
     GITBASE="develop"
 else
     # Get the reference, using the GitHub API
-    GITBASE=`curl -q https://api.github.com/repos/matrix-org/synapse/pulls/${CIRCLE_PR_NUMBER} | jq -r '.base.ref'`
+    GITBASE=`wget -O- https://api.github.com/repos/matrix-org/synapse/pulls/${CIRCLE_PR_NUMBER} | jq -r '.base.ref'`
 fi
 
 # Show what we are before
@@ -31,4 +31,4 @@ git fetch -u origin $GITBASE
 git merge --no-edit origin/$GITBASE
 
 # Show what we are after.
-git show -s
+git show -s