extract_sources works with Windows paths

Prior to this, backtrace lines were simply split by a single colon.

Unfortunately that is also the drive letter delimiter in Windows paths which resulted in a lot of empty source fragments of "C:0". ("C" from the drive letter and 0 from `"/path/to/rails/file.rb:16".to_i`)

Now the trace line is split by the first colon followed by some digits, which works for both Windows and Unix path styles.
On the other hand: The Ruby documentation states that a line number is always present (http://www.ruby-doc.org/core-2.1.5/Exception.html#method-i-backtrace) and demands that manually added trace lines must adhere to the specified format (http://www.ruby-doc.org/core-2.1.5/Exception.html#method-i-set_backtrace)