Revert "Some indentation fixes"

This reverts commit ee487214d207d7daee360695fa1b6ffd101a6447.
Revert "Added tests to about_arrays.rb in order to learn array substraction"

This reverts commit 6815f8a755f155ede0ae784dcb5728a2a978db5a.
Added array substraction koans to about_arrays.rb