travis: always build against tip

x packages are not guranteed to build against previous versions of
Go. The current build is failing because of:

../net/context/ctxhttp/ctxhttp.go:35: req.Cancel undefined

Change-Id: Ib7ee48455c1c2d04aa6b82e2202a3796fcf3478d
Reviewed-on: https://go-review.googlesource.com/23286
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

.travis.yml

@@ -1,8 +1,7 @@
 language: go
 
 go:
-  - 1.3
-  - 1.4
+  - tip
 
 install:
   - export GOPATH="$HOME/gopath"