prerequisites maven version parsing doesn't handle properties

[Patrick Roumanoff]

I have seen a pom with

<prerequisites>
  <maven>${maven.version}</maven>
</prerequisites>
<properties>
  <maven.version>3.0.5</maven.version>
</properties>

This break the logic when guessing the maven version by parsing the pom.xml file with ruby which doesn't know the first thing about maven properties.

mvnvm tries to look for ${maven.version} instead of 3.0.5