Add Go tip to coverage matrix

Also make this project compatible with running Travis CI on
forks - the go_import_path directive tells Travis to clone to
$GOPATH/src/github.com/davecgh/go-spew even if the remote user is
different.

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 19946dc..34a55c2 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,9 +1,11 @@
 language: go
+go_import_path: github.com/davecgh/go-spew
 go:
     - 1.6.x
     - 1.7.x
     - 1.8.x
     - 1.9.x
+    - tip
 sudo: false
 install:
     - go get -v github.com/alecthomas/gometalinter