Update version number to 0.7-pre.

There's a reason we didn't update this after 0.6 but I don't know
what it is. Let's see what breaks.
auto merge of #7418 : brson/rust/0.7-pre, r=cmr

There's a reason we didn't update this after 0.6 but I don't know
what it is. Let's see what breaks.