Missed a namespace for file (todo: why did TK not catch this)

- Fixes #10

Author: sethvargo

libraries/swapfile_provider.rb

@@ -159,7 +159,7 @@ def persist
 
             contents << addition
             contents.reject! { |line| line.strip.empty? }
-            File.open(fstab, 'w') do |f|
+            ::File.open(fstab, 'w') do |f|
               f.write(contents.join("\n") + "\n")
             end
           end