[Travis] Build the plugin in travis.

Now that the BUCK file has been added.

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index f8bc09c..6da06d4 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -33,4 +33,4 @@
 
 script:
   - ../buck/bin/buck build gerrit > buck.log
-
+  - ../buck/bin/buck build plugins/automerge-plugin:automerge-plugin >> buck.log