Remove tip

Many failed builds, not really helpful for PRs right now.
diff --git a/.travis.yml b/.travis.yml
index bd9e4c6..325b90a 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -6,7 +6,6 @@
   - 1.3
   - 1.4
   - 1.5
-  - tip
 script: script/cibuild
 after_success:
   - go get golang.org/x/tools/cmd/cover