Exclude Ruby 2.4 / Active Record 4.2 from Travis for now

The currently released version fails because of a version of the json
gem that is incompatible with Ruby 2.4. We will add this back once an
updated version of Active Record 4.2 is released.

Signed-off-by: Grant Hutchins <ghutchins@pivotal.io>

Author: Jeremy Gustine

.travis.yml

@@ -17,6 +17,8 @@ env:
 
 matrix:
   allow_failures:
+    - rvm: "2.4.0"
+      env: ACTIVE_RECORD_VERSION="~> 4.2.0"
     - env: ACTIVE_RECORD_BRANCH="master"
     - env: ACTIVE_RECORD_BRANCH="5-0-stable"
     - env: ACTIVE_RECORD_BRANCH="4-2-stable"