Travis: fix bundler

Author: harding

.travis.yml

@@ -9,9 +9,9 @@ sudo: false
 ## Save bundler deps.  Requires sudo: false
 cache: bundler
 
-## Disable external link checking to prevent spurious failures because
-## of other people's downtime. This can also avoid wasting their
-## bandwidth.
+## Required if our Ruby version is 2.2 or below
+before_install:
+   - gem install bundler -v '< 2'
 
 ## Build all possible jekyll pages for use with link checking.
 script: