A few small tweaks:
- Simplify makefile
- Cancel in-progress CI jobs when new commits are pushed to a non-main
branch
- Drop manual codeql config now that GitHub supports automagical
configuration

This adds a new /hostname endpoint as originally proposed in #66. In
this implementation, it exposes a dummy hostname by default, and only
exposes the real hostname (via `os.Hostname()`) if the
`-use-real-hostname` flag is given on the command line.

* Fix installation of deps

* Rebuild static assets w/ newest go-bindata

* Update go versions for CI

* Fix deploy on merge

* Bump golangci-lint version

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.

 - automatic dependency installation
 - add linting
 - test w/ race detector by default