escape backslash twice to get the intended effect in backtrace suppression

https://github.com/jimweirich/rake/commit/e963112dfea1ae902784468894c93c460331abb3 subbed single backslashes with forward slashes, which results in odd behavior when Backtrace is in an installed gem as the path has dots in it, which are escaped.

This changes it to replace `\\` with `/` 

to reproduce, install the latest version of rake and run a failing test suite using the test task. You'll see a long stack trace like in jimweirich/rake#23