travis: drop go1.7 and go1.8 due to golang.org/x/sys

Ref: #43 (comment)

golang.org/x/sys as of fde4db37ae7ad8191b03d30d27f258b5291ae4e3 uses
`math/bits`, so it does not support go1.7 and go1.8
I could set that in the go.mod, but those old versions do not support
that. I could ./vendor/ but perhaps no one is even using this tool on
these old golang versions. Choices. choices.

Signed-off-by: Vincent Batts <[email protected]>

Author: vbatts

.travis.yml

@@ -8,8 +8,6 @@ go:
   - "1.11.x"
   - "1.10.x"
   - "1.9.x"
-  - "1.8.x"
-  - "1.7.x"
 
 env: