Fix travis CI

Update the travis CI config to fix emacs setup in it.

Also add a job for Emacs 26.

.travis.yml

@@ -1,6 +1,14 @@
 # Config file copied from https://github.com/flycheck/emacs-travis
 language: emacs-lisp
-sudo: false
+# Emacs fails to build in container-based builds on Travis
+# See https://github.com/travis-ci/travis-ci/issues/9061
+# and https://github.com/moby/moby/issues/22801
+sudo: required
+dist: trusty
+cache:
+  - directories:
+      # Cache stable Emacs binaries (saves 1min per job)
+      - "$HOME/emacs/"
 # Allow Emacs snapshot builds to fail and don’t wait for these as they can take
 # a looooong time
 matrix:
@@ -11,6 +19,7 @@ env:
   - EMACS_VERSION=24.3
   - EMACS_VERSION=24.5
   - EMACS_VERSION=25.1
+  - EMACS_VERSION=26.1
   - EMACS_VERSION=snapshot
 before_install:
   # Configure $PATH: Executables are installed to $HOME/bin