Update Buck to recent version

This version fixes this bug [1] that allows us to eliminate these ugly
shell backticks and switch again to escaped macro like invocations:

  \$(dirname $OUT)

[1] https://github.com/facebook/buck/issues/212

Change-Id: Ie43b3ff6dfc89c4d840f2ff75c64ac0a5c6346b6

tools/default.defs

@@ -80,7 +80,7 @@ def gerrit_plugin(
     type = 'plugin',
     visibility = ['PUBLIC']):
   from multiprocessing import cpu_count
-  mf_cmd = 'v=`git describe HEAD`;'
+  mf_cmd = 'v=\$(git describe HEAD);'
   if manifest_file:
     mf_src = [manifest_file]
     mf_cmd += 'sed "s:@VERSION@:$v:g" $SRCS >$OUT'