"Normalize" most calls to getMeteorReleaseVersion

ie, make it prepend METEOR@ if necessary.

(We DON'T do that in the pre-springboard call in main because that one
might be telling us to springboard to a pre-0.9.0 release!)

Solves a bunch of issues like the previous commit where, if
.meteor/release just says "0.9.4", you get weird behavior.