Disable spec failing because of case insensitivity on Mac OSX

I am not sure how this spec ever passed.

Apparently `[ -f Requirements.txt]` always matches on Mac OSX (case
insensitive file system), even if only `requirements.txt` is present.
Therefor this spec will always fail because the bash script runs the
first branch of the `if` statement, not the second one.
Refactor towards sexp changes

Some refactorings and cleanup in order to work towards re-applying
the sexp changes.

This mostly makes all `Build::Script` implementations use
`sh.[dsl_method]` and stops including `Dsl` to `Build::Script`. Instead
`sh` will always be the current `Shell::Node`.