Allow failures on the snapshot

The the snapshot we use is constantly erroring the build without ever
running tests.

Author: Malabarba

.travis.yml

@@ -2,6 +2,9 @@ language: emacs-lisp
 env:
   - EMACS=emacs24
   - EMACS=emacs-snapshot
+matrix:
+  allow_failures:
+    - env: EMACS=emacs-snapshot
 before_install:
   # Stable Emacs 24.3
   - sudo add-apt-repository -y ppa:cassou/emacs