Remove unsupported versions (2.6 and 3.0)

See https://www.redmine.org/projects/redmine/wiki/Download (only 3.3, 3.2, and 3.1 are suggested for download)

Author: tianon

.travis.yml

@@ -5,8 +5,6 @@ env:
   - VERSION=3.3
   - VERSION=3.2
   - VERSION=3.1
-  - VERSION=3.0
-  - VERSION=2.6
 
 install:
   - git clone https://github.com/docker-library/official-images.git ~/official-images