update go version for example test run in readme

[fgimenez]

Hello! Following the readme I get an error with the suggested command for running the tests in a docker container with a previous version of Go, it happens to be 1.7, not the current previous one:

$ ./run_tests.sh 1.7
[...]
server.go:1282:11:warning: Until not declared by package time (gosimple)
server.go:1282:11:warning: Until not declared by package time (gosimple)
server.go:1282:11:warning: Until not declared by package time (unconvert)
server.go:1282:11:warning: Until not declared by package time (unconvert)
+ '[' 0 '!=' 0 ']'
+ go install . ./cmd/...
# github.com/decred/dcrd
./server.go:1282: undefined: time.Until

Not sure if this is intended, I've proposed changes for suggesting the previous, supported version of Go, in this case 1.8, which gives some warnings but doesn't fail.

Cheers!

[davecgh]

Thanks!