Fix Travis build

The plugin is only compatible with the Gerrit branch 2.10 for the
moment.

Change-Id: I3ecf2c4f3c321df3d8b7e1e0bf7888c77c14cf44

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index dfb6c36..15abc33 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -26,12 +26,13 @@
   - ant
   - export PATH=$PATH:$TRAVIS_BUILD_DIR/../../build/buck/bin
   - cd ..
-  - git clone https://gerrit.googlesource.com/gerrit
+  - git clone -b stable-2.10 https://gerrit.googlesource.com/gerrit
   - cd gerrit
   - ln -s $TRAVIS_BUILD_DIR plugins
   - travis_retry tools/download_all.py > download.log
 
 script:
+  - buck clean
   - buck build gerrit > buck.log
   - buck build plugins/automerge-plugin:automerge-plugin >> buck.log