Fixes deprecated File.exists?

Related info: http://www.ruby-doc.org/core-2.1.1/File.html#method-c-exist-3F (In "click to toggle source")

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Note: All recent pull requests fails probably due to:
```
/home/travis/.rvm/gems/ruby-1.9.3-p484/gems/activerecord-4.1.0/lib/active_record/connection_handling.rb:3:in `block in <module:ConnectionHandling>': undefined method `env' for Rails:Module (NoMethodError)
```
Since: rails/rails@88cd65b174d9a75d9cb0e5e06a57615ccc80fff1

:hatching_chick: