build: Remove old versions of Go on Travis CI

There is no real value in continuing to build against versions of Go
which are not the latest (1.10.2 right now). This can easily be
expressed in the `.travis.yml` file as `1.x`, to mean the latest version
in the Go 1 series, which is (presumably) what is being used for
releases.

Keeping older versions around is causing spurious CI failures, and
increases the cycle time to feedback for pull requests: see
hashicorp/packer#6265 for an example of this.

.travis.yml

@@ -6,11 +6,8 @@ sudo: false
 language: go
 
 go:
-  - 1.8.x
-  - 1.9.x
   - 1.x
 
-
 install:
   - make deps