internal: remove 1.6, 1.7, 1.8 from travis

Change-Id: I078a68bc9524f730d55543b5e25a97839ce01603
Reviewed-on: https://code-review.googlesource.com/c/35091
Reviewed-by: Eno Compton <enocom@google.com>
diff --git a/.travis.yml b/.travis.yml
index fe25d20..19fc065 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,9 +3,6 @@
 language: go
 
 go:
-  - 1.6.x
-  - 1.7.x
-  - 1.8.x
   - 1.9.x
   - 1.10.x
   - 1.11.x