travis: Remove go 1.8.x

vmg · vmg · commit c45d14f5a7c7 · 2019-03-12T16:16:30.000+01:00
According to the maintainer, Go 1.8 is no longer supported, so stop
testing it in Travis CI.
diff --git a/.travis.yml b/.travis.yml
@@ -1,7 +1,6 @@
 sudo: false
 language: go
 go:
-  - 1.8.x
   - 1.9.x
   - 1.10.x
   - 1.11.x
