Update Travis CI test matrix: test only oldest and most recent supported MRI releases, drop testing of unsupported Rails releases.

.travis.yml

@@ -3,10 +3,8 @@ language: ruby
 sudo: false
 
 rvm:
-  - 2.3.3
-  - 2.2.6
-  - 2.1.10
-  - 2.0
+  - 2.5
+  - 2.2
 
 env:
   - RAILS=5-2-stable DB=sqlite3
@@ -21,59 +19,7 @@ env:
   - RAILS=4-2-stable DB=mysql
   - RAILS=4-2-stable DB=postgres
 
-  - 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
-
 matrix:
-  exclude:
-    - rvm: 2.1.10
-      env: RAILS=5-2-stable DB=sqlite3
-    - rvm: 2.1.10
-      env: RAILS=5-2-stable DB=mysql
-    - rvm: 2.1.10
-      env: RAILS=5-2-stable DB=postgres
-
-    - rvm: 2.1.10
-      env: RAILS=5-0-stable DB=sqlite3
-    - rvm: 2.1.10
-      env: RAILS=5-0-stable DB=mysql
-    - rvm: 2.1.10
-      env: RAILS=5-0-stable DB=postgres
-
-    - rvm: 2.0
-      env: RAILS=5-2-stable DB=sqlite3
-    - rvm: 2.0
-      env: RAILS=5-2-stable DB=mysql
-    - rvm: 2.0
-      env: RAILS=5-2-stable DB=postgres
-
-    - rvm: 2.0
-      env: RAILS=5-0-stable DB=sqlite3
-    - rvm: 2.0
-      env: RAILS=5-0-stable DB=mysql
-    - rvm: 2.0
-      env: RAILS=5-0-stable DB=postgres
-
-    - rvm: 2.0
-      env: RAILS=4-2-stable DB=sqlite3
-    - rvm: 2.0
-      env: RAILS=4-2-stable DB=mysql
-    - rvm: 2.0
-      env: RAILS=4-2-stable DB=postgres
-
   include:
     - rvm: 2.3.3
       env: RAILS=master DB=sqlite3