'Command not found' when executing inside example directory

[melodell]

If you try to run the example from inside the example directory, the path to the executable doesn't resolve correctly and is_executable throws an error.

Reproduce with:

$ madoop --example
$ cd example
$ madoop \
  -input input \
  -output output \
  -mapper map.py \
  -reducer reduce.py
Error: Failed executable test: Command 'map.py' returned non-zero exit status 127.

(PS: We have a fix for this)