Enable API debug output by setting HUB_VERBOSE

This will print information about every HTTP request and response to
stderr. Authentication info is redacted.

Example:

    $ HUB_VERBOSE=1 hub ci-status
    > GET https://api.github.com/...
    > ...
Fix test failure on Ruby 1.8 due to hash order

Hashes are unordered in Ruby 1.8, so we can't check for this in debug
output: `{"name": "dotfiles", "private": false}`. The keys might appear
in different order.

A proper fix would be to normalize keys order before output assertion in
tests, but I can't be bothered to make that. Here I'm simply avoiding to
make any assertions for that line.

https://travis-ci.org/github/hub/jobs/19817554
Merge pull request #503 from github/verbose-api-output

Enable API debug output by setting HUB_VERBOSE