remove 1.10.x from travis matrix

go changed formatting behavior between .10 and .11 One option is to
download a specific version of gofmt but rather than do that we are
removing .10 tests.

Packer only supports the latest version of go, so it's unnecessary to
test on prior version. Previously we tested on .9 and .10 because go
changed the way tar encoding worked. We wanted to support .9 so users
would have a b/c option if needed. As we move forward that's become
unnecessary.

.travis.yml

@@ -6,7 +6,6 @@ sudo: false
 language: go
 
 go:
-  - 1.10.x
   - 1.11.x
   - master