Fix `hk log` under heroku-agent; don't mutate `http.DefaultTransport`

Unfortunately, having `HEROKU_AGENT_SOCK` set currently breaks `hk log`
because I overwrite the default `Dial` function. When `hk log` tries to
retrieve the URL for its log session, it can no longer perform a normal
HTTPS call properly.

This patches that issue by making `tr` just define a new transport which
it will then use for calls to the Heroku API and PG APIs. Even in
devclouds, setting `TLSClientConfig` should only be required for calls
to the API, so I don't _think_ that this will have any negative side
effects on the program.
Merge pull request #174 from heroku/brandur-fix-hk-log

Fix `hk log` under heroku-agent; don't mutate `http.DefaultTransport`