remove old go versions from travis

Author: nickysemenza

.travis.yml

@@ -3,20 +3,14 @@ language: go
 
 matrix:
   include:
-    - go: 1.12.x
-    - go: 1.13.x
     - go: 1.14.x
     - go: 1.15.x
-    - arch: s390x
-      go: 1.12.x
     - arch: s390x
       go: 1.13.x
     - arch: s390x
       go: 1.14.x
     - arch: s390x
       go: 1.15.x
-    - arch: ppc64le
-      go: 1.12.x
     - arch: ppc64le
       go: 1.13.x
     - arch: ppc64le