Use standard Tempfile naming to avoid Sprockets conflict

First off, many thanks for turning this into a gem, you've saved us a lot of time and headache. 

Regarding my changes- assuming that everything in https://github.com/rails/rails/issues/6989 is accurate, the monkey patch that Attachment Fu makes on `Tempfile` conflicts with Sprockets and causes the sporadic error I've been trying to track down. Fortunately, Ruby 1.8.7+ has its own mechanism for retaining file extensions when naming temporary files. I've removed the monkey patch and tweaked `random_tempfile_filename` to return an array as per the newer Tempfile initializer. 

In the interest of full disclosure I was not able to get the test suite running in full prior to making my changes (lots of dependencies, and it still thinks it's a plugin?), but have not encountered any issues thus far.