Dont print deprecations and preview warnings when OCTOKIT_SILENT is set.

Add octokit_warn wrapper around Kernal#warn.

Add OCTOKIT_SILENT=true to travis env config.
Merge pull request #447 from octokit/no-deprecation-messages-on-ci

Dont print deprecations and preview warnings when OCTOKIT_SILENT is set.
Document OCTOKIT_SILENT in the readme