creates a tmp file without ensuring the tmp directory exists

[hiredman]

if you were to, in your project.clj set the java tmp directory to point to a directory that doesn't exist, alembic throws a filenotfound exception. because alembic is used deep in the depths of various bits of tooling figuring out this error can take some doing. it is very easy to end up in this situation, if say you have a large test suite that does a lot of temp file munging, and to facilitate easy clean up you set the tmp dir to something like /tmp/foo and your os helpfully deletes the contents of /tmp on reboot.

it is crazy to me that File doesn't ensure the tmp exists, but I have no idea how to go about getting that fixed.