travis: Use newer infrastructure, but require sudo for additional disk space.

This is because as of Dec 12, 2017, Travis changed their container images and seem
to give slightly less disk space. Using a sudo image gives more disk space.

.travis.yml

@@ -1,8 +1,7 @@
 git:
   submodules: false
 language: scala
-# run on new infrastructure
-sudo: false
+sudo: required
 cache:
   apt: true
   directories: