fix #134 - include base_ref and head_ref in github context from the event json

pkg/runner/testdata/pull-request/event.json

@@ -0,0 +1,10 @@
+{
+  "pull_request": {
+    "head": {
+      "ref": "sample-head-ref"
+    },
+    "base": {
+      "ref": "sample-base-ref"
+    }
+  }
+}