travis: mark "git-snapshot" as allowed failure

Author: immerrr

.travis.yml

@@ -15,6 +15,10 @@ env:
   - EVM_EMACS=emacs-25.1-travis
   - EVM_EMACS=emacs-25.2-travis
   - EVM_EMACS=emacs-25.3-travis
+matrix:
+  allow_failures:
+    - env: EVM_EMACS=emacs-git-snapshot-travis-linux-xenial
+  fast_finish: true
 before_install:
   - curl -fsSkL https://gist.github.com/rejeep/ebcd57c3af83b049833b/raw > travis.sh && source ./travis.sh
   - evm install "$EVM_EMACS" --use --skip