Bump ClojureScript to 0.0-1978

I upped it from 1913 to 1934 at some point, but didn't commit. 0.0-1934
has a bug causing a spurious "Doesn't support name: 400" error on
partial rebuilds.
Partial fix for 180° turns (issue #3)

The other part is to advance the snake only by entire pixels at a time.
You can still trigger the bug if you are going right, press up/down,
"advance" 0.3333 pixels then press left, and the floor of your new
location equals the floor of the old. This is because using a position
variable with more precision than our collision-detecting routine is
silly :)