Rename __standalone_mode__ to STANDALONE_MODE

The fix for [1] prevents from importing python variables __foo__,
as explained in [2]. Rename standalone mode flag correspondingly.

[1] https://github.com/facebook/buck/issues/182
[2] https://github.com/facebook/buck/issues/221

Change-Id: I0ac05c80a1d5fc1b8f8bd2c5ca5dabdfeeecbe52

gerrit_plugin.bucklet

diff --git a/gerrit_plugin.bucklet b/gerrit_plugin.bucklet
index 7da239e..9d79ae4 100644
--- a/gerrit_plugin.bucklet
+++ b/gerrit_plugin.bucklet
@@ -53,7 +53,7 @@
   '//lib/gwt:dev',
 ]
 
-__standalone_mode__ = True
+STANDALONE_MODE = True
 
 def gerrit_plugin(
     name,