travis: update the Go versions

We need to use `runtime.KeepAlive()` which only exists past Go 1.7. Furthermore,
Go 1.7 is the latest supported by the language team.
diff --git a/.travis.yml b/.travis.yml
index f3b8b46..6131e6d 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,9 +1,8 @@
 language: go
 
 go:
-  - 1.5
-  - 1.6
   - 1.7
+  - 1.8
   - tip
 
 script: make test-static