Support cmd/go-httpbin (👍) and cmd/go_httpbin (👎)

In #17, we switched to deploying httpbingo.org on Google App Engine. To
meet their naming requirements, we had to rename cmd/go-httpbin to
cmd/go_httpbin, but this broke any existing uses of, e.g.,

    go get github.com/mccutchen/go-httpbin/cmd/go-httpbin

as suggested in the README and as used in various places (including my
employer).

Here's a dumb ass fix for that dumb ass problem; I'm not happy about it,
but it seems to work.