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.
Merge pull request #254 from mumoshu/fix-deprecation-warning

Fix deprecation warning
Merge pull request #262 from DylanLacey/master

Allow for special capabilities to be added by the end user.
Merge pull request #264 from DylanLacey/master

Individual Platforms, Multiple Files to Rake, PTests 1.0.1
Fixing tests
Preparing for release 3.5.0
Add details to readme for single platform caps
Version was set incorrectly from test_base
Have whitelist persist across all instances of Sauce.config