travis: run both Go versions on osx and windows

This simplifies build matrix maintenance and is paid for by the previous
removal of the third last Go version support tests.

.travis.yml

@@ -36,32 +36,22 @@ matrix:
  fast_finish: true
  exclude:
   - os: osx
-    go: 1.13.x
     env: TAGS="-tags bounds"
   - os: osx
-    go: 1.13.x
     env: TAGS="-tags noasm"
   - os: osx
-    go: 1.13.x
     env: TAGS="-tags appengine"
 
-  - os: osx
-    go: 1.12.x
   - os: osx
     go: master
 
   - os: windows
-    go: 1.13.x
     env: TAGS="-tags bounds"
   - os: windows
-    go: 1.13.x
     env: TAGS="-tags noasm"
   - os: windows
-    go: 1.13.x
     env: TAGS="-tags appengine"
 
-  - os: windows
-    go: 1.12.x
   - os: windows
     go: master
  allow_failures: