travis: update the ruby versions to test against

Remove those lower than 2.2 and add 2.5.

Author: carlosmn

.travis.yml

@@ -3,13 +3,13 @@ cache: bundler
 
 os:
   - linux
+  - osx
 
 rvm:
-  - 2.0.0
-  - 2.1.8
   - 2.2.5
   - 2.3.3
   - 2.4.0
+  - 2.5.0
   - ruby-head
   - rbx-2
 
@@ -28,15 +28,6 @@ matrix:
   allow_failures:
     - rvm: rbx-2
     - rvm: ruby-head
-  include:
-    - os: osx
-      rvm: 2.0.0
-    - os: osx
-      rvm: 2.1.5
-    - os: osx
-      rvm: 2.2.2
-    - os: osx
-      rvm: rbx-2
 
 git:
   submodules: false