Use sudo: true on travis ci (#5538)

* use sudo: true on travis ci

* minor comment to explain previous change

This relies on having 4 core 16 gigs Travis machines enabled for the account.

.travis.yml

@@ -4,7 +4,9 @@ language: c
 
 dist: trusty
 
-sudo: false
+# This sets the default config for each job to use full VMs.
+# The VMs have 2 cores and 8 gigs of ram. Larger VMs are also available.
+sudo: true
 
 # We whitelist branches, as we don't really need to build dev-branches.
 # Remember to add release branches, both here and to appveyor.yml.