Make "s" in "https" optional to match URL w/ regex

Some folks may be using a GitHub Enterprise server that doesn't support HTTPS.

Without this fix, I get errors when trying to use github-cli with our non-HTTPS-enabled GitHub Enterprise server.

```
$ gh -r devmonkeys/smstack issue.ls
Traceback (most recent call last):
  File "/Users/marca/python/virtualenvs/github-cli/bin/gh", line 9, in <module>
    load_entry_point('gh-cli==0.1.b', 'console_scripts', 'gh')()
  File "/Users/marca/dev/git-repos/github-cli/gh/main.py", line 29, in main
    status = commands[command].run(opts, args[1:])
  File "/Users/marca/dev/git-repos/github-cli/gh/commands/issue/ls.py", line 56, in run
    return self.print_issues(opts)
  File "/Users/marca/dev/git-repos/github-cli/gh/commands/issue/ls.py", line 86, in print_issues
    for i in issues:
  File "/Users/marca/python/virtualenvs/github-cli/lib/python2.7/site-packages/github3/structs.py", line 82, in __iter__
    yield cls(i, self) if issubclass(cls, GitHubCore) else cls(i)
  File "/Users/marca/python/virtualenvs/github-cli/lib/python2.7/site-packages/github3/issues/issue.py", line 83, in __init__
    self.repository = m.groups()
AttributeError: 'NoneType' object has no attribute 'groups'
```