(MAINT) Update linux distro in travis

This commit updates our travis config to use xenial,
16.04 instead of trusty, 14.04, because trusty has
reached EOL and is now causing errors.

Author: michaeltlombardi

.travis.yml

@@ -6,7 +6,7 @@ os:
   # will work with our PowerShell manager code.
   - osx
 
-dist: trusty
+dist: xenial
 language: ruby
 cache: bundler
 before_install: