Replace File.exists? with File.exist? to avoid deprecation warning

`File.exists?` is a deprecated name in the latest MRI `ruby-2.1.2` - see it on [ruby-doc](http://www.ruby-doc.org/core-2.1.2/File.html#method-c-exists-3F)

I started getting the warning with `ruby-2.1.0`. Here it is:

```
gems/state_machine-1.2.0/lib/state_machine/integrations/base.rb:81: warning: File.exists? is a deprecated name, use File.exist? instead
```