bbdispatch: forcing run against a single perl doesn't provide results

[stevieb9]

When using the -c option and specifying only a single perl to run against, no results are returned, presumably due to the version heading not being present.

bbdispatch -t localhost -r stevieb9/mock-sub -d 7 -c "brewbuild -o 5.28.1"

[stevieb9]

Very likely related to #241

[stevieb9]

This actually turned out not to be a problem, but another one emerged while troubleshooting.

What was happening, was I was typing an incorrect version to the -o argument, but BrewBuild wasn't handling that at all. It was just proceeding away.

We now check against all versions in the -o arg and validate them. If any of them are invalid, we croak().