Compile to /js/main to avoid trashing /js/lib.

When we compiled to just /dev-resources/public/js, `lein clean` would
wipe out the existing contents of the folder before compiling, so
/dev-resources/public/js/lib would get trashed. This might be perfectly
reasonable behavior, so rather than try to bend the shape of things, I
just named /dev-resources/public/js/main as the compile target, so other
folders can exist in /public/js in peace.