[travis] Ensure use of standard (legacy) infrastructure

- For 'sudo apt' commands on Linux (Mac unaffected)
- Seems already set for main QGIS repo, but now needed when adding
  Travis CI setup to a personal fork ("For repos we recognize on or
  after 2015-01-01, linux builds are sent to our container-based
  infrastructure")
- This could probably be overcome using the Travis 'apt' plugin so that
  container-based infrastructure could be used
  http://docs.travis-ci.com/user/apt/

http://docs.travis-ci.com/user/workers/standard-infrastructure/
http://docs.travis-ci.com/user/workers/container-based-infrastructure/

.travis.yml

@@ -1,5 +1,7 @@
 language: cpp
 
+sudo: required
+
 os:
   - linux
   - osx