Fix deprecation warning

`File.exists?` is deprecated according to
http://batsov.com/articles/2014/02/05/a-list-of-deprecated-stuff-in-ruby/

We should use `File.exist?` instead.

Without this fix, using Ruby 2.1.2 and sauce 3.4.9, I'm constantly getting warnings (like 100 warnings per spec) as follows:

```
/cache/bundle-install/ruby/2.1.0/gems/sauce-3.4.9/lib/sauce/config.rb:350: warning: File.exists? is a deprecated name, use File.exist? instead
/cache/bundle-install/ruby/2.1.0/gems/sauce-3.4.9/lib/sauce/config.rb:350: warning: File.exists? is a deprecated name, use File.exist? instead
<repeat>
```