tell octokit where the github instance is

useful for github enterprise instances
fixing typo
fixing hard-coded GitHub reference in issue hook
moving to GITHUB_ENTERPRISE_URL var
typos
changing env var to BOXEN_GITHUB_ENTERPRISE_URL
adding REPO_URL_TEMPLATE to config
fixing test method name
Merge branch 'master' into librarian-puppet-fix
Merge pull request #1 from mdelagralfo/librarian-puppet-fix

Librarian puppet fix
don't use GH:E API tokens to get public GH tarballs
Merge pull request #2 from mdelagralfo/librarian-puppet-fix-2

don't use GH:E API tokens to get public GH tarballs
adding 'enterprise?' method to boxen config
changing 'repourl' to 'repotemplate' in boxen config
use a URI instance to get the github hostname
Merge pull request #70 from mdelagralfo/ghe-alternate-env

Add support for GitHub Enterprise