travis: drop Go 1.7 and 1.8
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

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs