Rescue from NoMethodError when trying to load missing netrc

If you try to authenticate with `netrc` credentials and there's no entry in the `netrc` file for the given api endpoint, Octokit throws an unhelpful error:

```ruby
>> c = Octokit::Client.new netrc: true
NoMethodError: undefined method `shift' for nil:NilClass
	from /Users/clint/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/octokit-2.6.1/lib/octokit/authentication.rb:61:in `login_from_netrc'
	from /Users/clint/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/octokit-2.6.1/lib/octokit/client.rb:91:in `initialize'
	from (irb):4:in `new'
	from (irb):4
	from /Users/clint/.rbenv/versions/2.0.0-p247/bin/irb:12:in `<main>'
```

With this patch, output changed to this:

```ruby
>> c = Octokit::Client.new netrc: true
Error loading credentials from netrc file for https://api.github.com/
#<Octokit::Client:0x007f94052d1478 @access_token=nil, @api_endpoint="https://api.github.com"
```

This adds a simple `rescue NoMethodError` with a `warn` to the user. I'm all for suggestions on handling this better than the `NoMethodError` catch, or the wording in the `warn` output.