Move back to attohttpc, now that it supports proxies

[charlespierce]

Info

• In Switch to reqwest for proxy support #809, we switched to reqwest for handling HTTP requests to support proxies via HTTP_PROXY and HTTPS_PROXY environment variables.
• However, that change came with an increase in build time and binary size, since reqwest is a much larger library than attohttpc.
• Additionally, reqwest currently doesn't support the NO_PROXY environment variable for carving out exceptions to the proxy environment.
• attohttpc recently (with version 0.16.0) added support for proxies, including the NO_PROXY environment variable.

Changes

• Switched back to using attohttpc, but requiring version 0.16.0.
• This is mostly a revert of Switch to reqwest for proxy support #809, accounting for merge conflicts.

Tested

• All existing automated tests work.
• Local testing to confirm that proxy environment variables are supported.
• Local testing to confirm that NO_PROXY is respected as well.

[chriskrycho]

Nice reversion, quick turnaround on this thing. I like it!