CI: simplify config

- Nowadays `sudo false` is not needed anymore.
- Updating Homebrew takes much longer than running the actual tests. Just use
  the old Vim version installed by default on macOS.

.travis.yml

@@ -1,5 +1,3 @@
-sudo: false
-
 os:
   - linux
   - osx
@@ -9,8 +7,5 @@ addons:
     packages:
       - vim-gtk
 
-before_script:
-  - if [[ "$TRAVIS_OS_NAME" == osx ]]; then brew update && brew install vim; fi
-
 script:
   - test/run