Skip go generate check on older versions

runtests

@@ -70,6 +70,11 @@ def _toc():
 
 
 def _gen():
+    go_version = check_output('go version'.split()).split()[2]
+    if go_version < 'go1.4':
+        print('runtests: skip on {}'.format(go_version), file=sys.stderr)
+        return
+
     _run('go generate .')
     _run('git diff --quiet')