Per discussion in #1125 (comment), we'd like to consider a few potential changes to how gtfs-validator handles version numbers. Two potential issues:

• The use of a leading v in the version tag (ala v3.1.0) causes problems for some tooling (Java Modules, Mac OS app packaging) that expects numeric major.minor.patch version numbers.
• Our use of a versionTag environment variable for passing in version information is maybe not idiomatic Gradle? My limited research has suggested that overriding project.version via a command-line flag ala -Pversion=1.2.3 is a more consistent way of setting the project version for the entire build.