travis: drop Go 1.7 and 1.8

Fails with

get "golang.org/x/crypto/scrypt": verifying non-authoritative meta tag
package math/bits: unrecognized import path "math/bits" (import path does not begin with hostname)

Other projects have dropped the old Go version for the same
reason, example: https://github.com/nmrshll/gphotos-uploader-cli/issues/7

.travis.yml

@@ -10,8 +10,6 @@ git:
 # Build with the lastest versions of Go 1.7 and later
 # See https://golang.org/dl/
 go:
-  - 1.7.x
-  - 1.8.x
   - 1.9.x
   - 1.10.x
   - 1.11.x