internal: deprecate 1.6-1.8 (#56)

diff --git a/.travis.yml b/.travis.yml
index f931fc2..cc0a91e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,8 +1,9 @@
 sudo: false
 language: go
 go:
-  - 1.6
-  - 1.7
+  - 1.9.x
+  - 1.10.x
+  - 1.11.x
 script:
   - gofmt -l .
   - go tool vet .