Add missing require of fileutils

[areitz]

While using Subprocess.check_output with the cwd: option, I encountered this exception:

17:28:58 /var/lib/gems/2.1.0/gems/subprocess-1.3.0/lib/subprocess.rb:253:in `block in initialize': uninitialized constant Subprocess::Process::FileUtils (NameError)
17:28:58    from /var/lib/gems/2.1.0/gems/subprocess-1.3.0/lib/subprocess.rb:251:in `fork'
17:28:58    from /var/lib/gems/2.1.0/gems/subprocess-1.3.0/lib/subprocess.rb:251:in `initialize'
17:28:58    from /var/lib/gems/2.1.0/gems/subprocess-1.3.0/lib/subprocess.rb:85:in `new'
17:28:58    from /var/lib/gems/2.1.0/gems/subprocess-1.3.0/lib/subprocess.rb:85:in `check_output'
17:28:58    from /pay/jenkins/stripe/scripts/postprocess-coverage.rb:68:in `main'
17:28:58    from /pay/jenkins/stripe/scripts/postprocess-coverage.rb:73:in `<main>'

Line 253 uses the FileUtils module, but unless this module has already been brought into the global namespace, this call will fail.

Incidentally, the unit test

https://github.com/stripe/subprocess/blob/master/test/test_subprocess.rb#L185

Succeeds because the test file requires tempfile, which transitively brings fileutils into the global namespace.

[zenazn]

Good find—LGTM!