Extract regular expression for app versions.
Expand version RE to accept all legal semver versions.
Make get_*_suffix module-private.
Ensure that if VERSION is semver, then autonumbers we create are too.
Merge pull request #29 from dnanexus/PTFM-8115-semver-for-apps

PTFM-8115 dx-build-app allows version to be any semver-valid version.