v1.1

Quick bugfix update, thanks to everyone who reported these:

- Removed the ability to pick up time-reversed crates through thin platforms. This was meant as a time-saving feature, so that levels could be beaten with slightly less repetitive keypresses, but several people reported it as a bug since it doesn't make sense within the model of time travel the game uses.
- Redesigned level 20 completely as it had a solution that did not require one of the boxes. The new design is slightly harder, so I have also swapped it with level 21 to smooth out the difficulty curve.
- Level 18 has been reworked as it was a little confused.