Follow core's switch from parboiled to grappa

Core switched from parboiled to grappa in

  0db7612e092b37c6ea04883a5a45f51c6d9ae433

to easy the gittiles plugin. Since we used one of parpoiled's helpers,
we have to follow the switch to grappa.

Change-Id: I95f90bb9abc493c7386a587b14c99da78251de25
diff --git a/BUCK b/BUCK
index c5d6009..29197f0 100644
--- a/BUCK
+++ b/BUCK
@@ -6,7 +6,9 @@
   '//lib/guice:guice',
   '//lib/jgit:jgit',
   '//lib/log:api',
-  '//lib:parboiled-core',
+  '//lib:grappa', # used for it's parboiled part. See
+  # core's 0db7612e092b37c6ea04883a5a45f51c6d9ae433
+  # for details
   '//lib:velocity',
 ]