Update to dynapath 0.2.3.

This is primarily to prevent conflicts with other deps that use the
zero-arity version of `all-classpath-urls`, which was introduced in
0.2.3, but also brings in a fix that prevents modification of the boot
class loader.

This change shouldn't impact Midje code at all, and all of the tests
pass for me with this change.
Merge pull request #268 from tobias/dynapath-update

Update to dynapath 0.2.3.