Fix commits check, commits not always present

Fixes #76

Author: knownasilya

lib/webhooks.js

@@ -47,7 +47,7 @@ function makeJob(project, config) {
 
 function startFromCommit(project, payload, send) {
   var config = pushJob(payload);
-  var lastCommit = payload.commits[payload.commits.length - 1];
+  var lastCommit = payload.commits ? payload.commits[payload.commits.length - 1] : false;
 
   if (!lastCommit) {
     lastCommit = {};