travis: use Go 1.7.4, drop old env var

We don't use Go 1.5.x anymore

.travis.yml

@@ -4,12 +4,10 @@ go_import_path: github.com/coreos/etcd
 sudo: false
 
 go:
-  - 1.7.3
+  - 1.7.4
   - tip
 
 env:
-  global:
-   - GO15VENDOREXPERIMENT=1
   matrix:
    - TARGET=amd64
    - TARGET=arm64