Make the exec backend return proper exitstatus.

Before this pull request, `Serverspec::Backend::Exec#run_command` returns `$?` as exit status.
`$?` is an instance of `Process::Status`.

If `$?` is compared with an interger, `$?.to_i` will be compared with the value.
`Process::Status#to_i` doesn't return exit status but returns the bits in the status as Fixnum.

c.f.) http://www.ruby-doc.org/core-2.0.0/Process/Status.html