Fix according to specinfra 2.0.0beta18
Use Specinfra.command to get command
Use change_file_mode and change_file_owner instead of calling
run_command directly
Update dependency
Use set method to set backend type
Fix spec
Merge pull request #7 from mizzy/fix-for-specinfra-b2-beta18

Fix for specinfra 2.0.0beta18