Travis CI: Try "dist: trusty" + "sudo: required"

This is the only combination not yet tested, see
https://github.com/travis-ci/travis-ci/issues/8361#issuecomment-327817604

Test it, although it does not look very promising.

.travis.yml

@@ -13,8 +13,8 @@
 dist: trusty
 
 
-sudo: false      # dist=precise + sudo=false may be discontinued
-#sudo: required   # sudo is not required, keep the default
+#sudo: false      # dist=precise + sudo=false may be discontinued
+sudo: required   # sudo is not required, keep the default
 
 
 ##############################################################################