Allow failure of ~> 3.2.17 for the moment.

This is because the security release of `3.2.22.1` was not correctly
tested against 1.8.7 which means that our build will not pass against
this ruby. /cc @pixeltrix.

Author: Sam Phippen

.travis.yml

@@ -104,6 +104,8 @@ matrix:
     - rvm: 2.3.0
       env: RAILS_VERSION='~> 3.1.12'
   allow_failures:
+    - rvm: 1.8.7
+      env: RAILS_VERSION='~> 3.2.17'
     - rvm: 2.2.2
       env: RAILS_VERSION=master
     - rvm: 2.3.0