Remove awful post_install_message.

It's awful. And it doesn't just effect people who choose to use this library. It blights the output of `bundle` even when it's a second or Nth-level dependency.

Others including @raykrueger @h3h @nbibler @olivierlacan @lparry @tobico @eric @tomdale @locks @shime @nt1 @avdi @christhekeele @markijbema @marten @harto agree:

* https://github.com/jnunemaker/httparty/pull/139
* https://github.com/jnunemaker/httparty/pull/211
* https://github.com/jnunemaker/httparty/pull/236
* https://github.com/jnunemaker/httparty/pull/237
* https://github.com/jnunemaker/httparty/pull/294
* https://github.com/jnunemaker/httparty/pull/315

I'll keep trying to solve this downstream by encouraging other libraries to not use httparty, but it'd be great to fix it centrally.