Add fake Github Homebrew Token for Travis

We recently added a new homebrew nightly builder, which pushes to the
`servo/homebrew-servo` repository to make new binary nightlies
available via Homebrew. This requires a Github token to push to the
repository.

However, the original PR did not include a fake token for the Travis
failure, causing the builder to fail only on Travis (where the test
pillars did not have the corresponding token), but not in production.

.travis/test_pillars/buildbot/master.sls

@@ -4,6 +4,7 @@ buildbot:
     'http-pass': 'TEST_BUILDBOT_HTTP_PASS'
     'change-pass': 'TEST_BUILDBOT_CHANGE_PASS'
     'gh-doc-token': 'TEST_BUILDBOT_GH_DOC_TOKEN'
+    'gh-homebrew-token': 'TEST_BUILDBOT_GH_HOMEBREW_TOKEN'
     'gh-hook-secret': 'TEST_BUILDBOT_GH_HOOK_SECRET'
     'gh-status-token': 'TEST_BUILDBOT_GH_STATUS_TOKEN'
     'homu-secret': 'TEST_BUILDBOT_HOMU_SECRET'