Update travis config

And fix 26.1 build (see rejeep/evm#125)

.travis.yml

@@ -1,4 +1,5 @@
 language: generic
+dist: trusty
 sudo: false
 branches:
   only:
@@ -15,6 +16,10 @@ before_install:
 env:
   - EVM_EMACS=emacs-25.3-travis
   - EVM_EMACS=emacs-26.1-travis
+  - EVM_EMACS=emacs-git-snapshot-travis
+matrix:
+  allow_failures:
+  - env: EVM_EMACS=emacs-git-snapshot-travis
 script:
   - bin/doom version
   - bin/doom -d test