WARNING!!!
Doing this rollback will corrupt your save data!
SORRY!! v0.4.5 is way more broken than we thought. We've had reports of players not even being able to start levels, so we've put v0.4.4 on the rollback branch.
You can access the rollback branch with the password "polyrollback" (without quotes of course). Here are some more instructions, under Product Betas, if you aren't familiar with the process.
Again, sorry about the brokenness. We'll get a fix-er-up patch out sometime this week.
EDIT: Another sorry!! Your save data will wiped if you do the rollback, and won't be recoverable. Very bad oversight by me. Any new save data will not be lost when updating though.