remove some too old versions from travis

Author: c0b

Diff for: .travis.yml

@@ -21,14 +21,6 @@ env:
   - DIR=1.7 VARIANT=alpine
   - DIR=1.7 VARIANT=otp-21.2
   - DIR=1.7 VARIANT=otp-21.2-alpine
-  - DIR=1.6
-  - DIR=1.6 VARIANT=slim
-  - DIR=1.6 VARIANT=alpine
-  - DIR=1.6 VARIANT=otp-21
-  - DIR=1.6 VARIANT=otp-21-alpine
-  - DIR=1.5
-  - DIR=1.5 VARIANT=slim
-  - DIR=1.5 VARIANT=alpine
 
 install:
   - curl -fsSL https://github.com/docker-library/official-images/archive/master.tar.gz | {