Update Travis CI test matrix: stop testing agaist Rails master.

.travis.yml

@@ -19,27 +19,6 @@ env:
   - RAILS=4-2-stable DB=mysql
   - RAILS=4-2-stable DB=postgres
 
-matrix:
-  include:
-    - rvm: 2.3.3
-      env: RAILS=master DB=sqlite3
-    - rvm: 2.3.3
-      env: RAILS=master DB=mysql
-    - rvm: 2.3.3
-      env: RAILS=master DB=postgres
-
-    - rvm: 2.2.6
-      env: RAILS=master DB=sqlite3
-    - rvm: 2.2.6
-      env: RAILS=master DB=mysql
-    - rvm: 2.2.6
-      env: RAILS=master DB=postgres
-
-  allow_failures:
-    - env: RAILS=master DB=sqlite3
-    - env: RAILS=master DB=mysql
-    - 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%";'