Binary search tree exercise checks for version on Bst class instead of Bookkeeping module

[Ryan1729]

The binary search tree exercise test checks for Bst::VERSION instead of BookKeeping::VERSION like the other exercises.

Not the biggest deal, but it's probably worth fixing if/when #513 is addressed.

[kotp]

If you would like, you can do the pull request regardless of the status of #513

[Ryan1729]

Ah, I wasn't sure if the exercise was generated or not, but given the absence of a canonical-data.json file in the problem-specifications repo I suppose it isn't.