replaced testing on tip to testing on latest stable

Signed-off-by: Josh Hill <jhill@pivotal.io>

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 3b26c7c..d65d227 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,7 @@
 language: go
 go:
   - 1.5
-  - tip
+  - stable
 
 install:
   - go get -v ./...