Bump test go patchlevels (#113)

* 1.6.4
* 1.7.4

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 7f11d82..82e1775 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,8 +1,8 @@
 language: go
 go:
   - 1.5.4
-  - 1.6.3
-  - 1.7.3
+  - 1.6.4
+  - 1.7.4
   - tip
 matrix:
   allow_failures: