Use File.open instead of File.new with block.

Since File::new() does not take block, we should use File.open instead of File.new with block.

Warning example:
```
Chef::Util::FileEdit
  initialiize
/Users/ryota/code/github/opscode/chef/lib/chef/util/file_edit.rb:36: warning: File::new() does not take block; use File::open() instead
/Users/ryota/code/github/opscode/chef/lib/chef/util/file_edit.rb:36: warning: File::new() does not take block; use File::open() instead
    should create a new Chef::Util::FileEdit object
/Users/ryota/code/github/opscode/chef/lib/chef/util/file_edit.rb:36: warning: File::new() does not take block; use File::open() instead
    should throw an exception if the input file does not exist
/Users/ryota/code/github/opscode/chef/lib/chef/util/file_edit.rb:36: warning: File::new() does not take block; use File::open() instead
    should throw an exception if the input file is blank
```