Upgrade client_golang to v0.8.0 or newer

[matthiasr]

statsd_exporter is already using it, and with Go modules this prevents us from bringing in a newer version of the mapper.

There are also performance improvements in client_golang, so this will probably help with #66 as well.

[beorn7]

Note that some of the performance improvements are in prometheus/common, which is un-versioned so far and needs to be updated separately from client_golang.

[matthiasr]

fixed by #77