Fetch the gh-pages branch first

.travis.yml

@@ -11,6 +11,7 @@ before_install:
 - echo "IdentityFile ~/.ssh/deploy-key" >> ~/.ssh/config
 - echo "User git" >> ~/.ssh/config
 - git remote set-url origin git@github.com:mozilla/stab-crashes.git
+- git fetch origin -f gh-pages:gh-pages
 install:
 - pip install -r requirements.txt
 - pip install -r test-requirements.txt