Revert "temporary disable 1.6 on travis (#1198)" (#1199)

This reverts commit 72b617aacebed7836e806925e6f61eef55cb40c8.
diff --git a/.travis.yml b/.travis.yml
index 51f7d32..b3577c7 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,6 +1,7 @@
 language: go
 
 go:
+  - 1.6.3
   - 1.7
   - 1.8