Update file related things

I've updated specinfra like these.

* Add copy command to file resource (serverspec/specinfra#196)
* Rename copy_file to send_file in backend class, becayse it's confusing with copy command of file resource above (serverspec/specinfra#195)

In short,

* send_file
  * With exec backend, it means copying file locally
  * With ssh backend, it means transfering file to remote host
* copy_file
   * It means copying file locally both with exec and ssh backend

Please check it and ask me if you have any questions.