drop CI for Ruby 2.2, Rails 5.0

.travis.yml

@@ -4,7 +4,6 @@ sudo: false
 
 rvm:
   - 2.5
-  - 2.2
 
 env:
   - RAILS=5-2-stable DB=sqlite3
@@ -23,10 +22,6 @@ env:
   - RAILS=5-1-stable DB=mysql
   - RAILS=5-1-stable DB=postgres
 
-  - RAILS=5-0-stable DB=sqlite3
-  - RAILS=5-0-stable DB=mysql
-  - RAILS=5-0-stable DB=postgres
-
 matrix:
   allow_failures:
     - env: RAILS=5-2-stable DB=sqlite3