travis: Remove all travis sources.list files early.

This fixes most of the current Travis CI failures, and should help
prevent future ones from cropping up, since we're just deleting Travis
CI's giant library of apt sources up front.

The production build is still failing, since apparently some of the
packages we were doing `apt-mark hold` on no longer exist.

.travis.yml

@@ -2,8 +2,9 @@
 # high-level documentation on our Travis CI setup.
 dist: trusty
 install:
-  # Disable broken riak sources.list in Travis base image 2017-10-18
-  - rm -vf "/etc/apt/sources.list.d/*riak*"
+  # Disable sometimes-broken sources.list in Travis base images
+  - sudo rm -vf /etc/apt/sources.list.d/*
+  - sudo apt-get update
 
   # Disable Travis CI's built-in NVM installation
   - mispipe "mv ~/.nvm ~/.travis-nvm-disabled" ts