.travis.yml: add sudo: required

Avoid Travis CI error for zsh on linux:

   $ ./test/support/travisci_deps.sh
   This job is running on container-based infrastructure, which does not
   allow use of 'sudo', setuid, and setgid executables.  If you require
   sudo, add 'sudo: required' to your .travis.yml

.travis.yml

@@ -6,6 +6,8 @@ env:
   - TEST_SHELLS=bash
   - TEST_SHELLS=zsh
 
+sudo: required
+
 install:
   - ./test/support/travisci_deps.sh