Update travis-ci for Rails 5.0.0 and 4-2-stable

Arel and Active Record have breaking changes in Rails 5.0.0 and we’ll
need to add an adapter for compatibility.

The Mongoid gem has not yet updated its dependancies for Rails 5.

.travis.yml

@@ -10,27 +10,38 @@ rvm:
   - 1.9
 
 env:
-  - RAILS=master DB=mongodb
-  - RAILS=master DB=sqlite3
-  - RAILS=master DB=mysql
   - RAILS=master DB=postgres
+
+  - RAILS=4-2-stable DB=mongodb
+  - RAILS=4-2-stable DB=sqlite3
+  - RAILS=4-2-stable DB=mysql
+  - RAILS=4-2-stable DB=postgres
+
   - RAILS=4-1-stable DB=mongodb
   - RAILS=4-1-stable DB=sqlite3
   - RAILS=4-1-stable DB=mysql
   - RAILS=4-1-stable DB=postgres
+
   - RAILS=4-0-stable DB=sqlite3
   - RAILS=4-0-stable DB=mysql
   - RAILS=4-0-stable DB=postgres
+
   - RAILS=3-2-stable DB=sqlite
   - RAILS=3-2-stable DB=mysql
   - RAILS=3-2-stable DB=postgres
+
   - RAILS=3-1-stable DB=sqlite
   - RAILS=3-1-stable DB=mysql
   - RAILS=3-1-stable DB=postgres
+
   - RAILS=3-0-stable DB=sqlite
   - RAILS=3-0-stable DB=mysql
   - RAILS=3-0-stable DB=postgres
 
+matrix:
+  allow_failures:
+    - env: RAILS=master DB=postgres
+
 before_script:
   - mysql -e 'create database ransack collate utf8_general_ci;'
   - mysql -e 'use ransack;show variables like "%character%";show variables like "%collation%";'