Test against same version of Ruby that GitHub Pages uses (#132)

Merge pull request 132

Author: pathawks

.travis.yml

@@ -7,7 +7,7 @@ cache: bundler
 matrix:
   include:
     - # GitHub Pages
-      rvm: 2.1.1
+      rvm: 2.4.0
       env: GH_PAGES=true
   allow_failures:
     - env: GH_PAGES=true # Jekyll 2.4 will fail tests