Make parser more robust against changes in the output of `rustc`

[Kimundi]

Right now the parser of rustcs version information is relatively strict, expecting a clearly defined format of lines.

However, seeing how rustc is unlikely to break its output in a backwards-incompatible way, but might add additional fields (as has already happened in the past), it might be more forward-compatible to just look for the "<key>: <value>" lines this crate knows about and silently ignore the rest.

[alexcrichton]

FWIW this caused this change in rustc output to bounce in the past