Update the backtrace parser for OCaml 4.11.0

[Octachron]

In OCaml 4.11, the backtrace has been updated to add function names whenever it is possible. For instance, in 4.11, a backtrace may look like

Called from OUnitRunner.run_one_test.(fun) in file "src/lib/ounit2/advanced/oUnitRunner.ml", line 83, characters 13-26

compared to

Called from file "src/lib/ounit2/advanced/oUnitRunner.ml", line 83, characters 13-26

in previous versions.

This change unfortunately breaks ounit backtrace parser.

This small PR proposes to update the parser to keep up with the improved backtrace messages.
The new Scanf format string is too clever for its own good but it works on all OCaml versions.

Close #17

[kit-ty-kate]

Should this make all ounit releases previous to 2.2.3 incompatible with OCaml 4.11 or just on a per usage basis? I don't know how wildly this function used in ounit itself and outside.

[gildor478]

This is an optional feature that helps you to jump back where the test is defined. It is mostly for human.

Do you have bug report of people not being able to use the feature with older version of OCaml?