Lock Travis to bundler 1.x (#341)

Bundler 2.0 has been released but has a dependency on rubygems 3.0, which is not available by default with our version of Ruby. So this PR attempts to lock Travis into using 1.x instead, as otherwise our build fails.

.travis.yml

@@ -34,6 +34,7 @@ before_install:
   - sudo apt-get install xvfb -y
   - sudo apt-get install ttf-liberation -y
   - sudo apt-get install wkhtmltopdf -y
+  - gem install -v 1.17.2 bundler --no-rdoc --no-ri
 before_script:
   # Set up Mongo databases
   - mongo waste-carriers-test --eval 'db.createUser({user:"mongoUser", pwd:"password1234", roles:["readWrite", "dbAdmin", "userAdmin"]})'