travis.yml: remove go 1.4 builder

.travis.yml

@@ -3,9 +3,6 @@ go_import_path: github.com/ethereum/go-ethereum
 sudo: false
 matrix:
   include:
-    - os: linux
-      dist: trusty
-      go: 1.4.2
     - os: linux
       dist: trusty
       go: 1.5.4