Use composer/semver internally to verify PHP version compatibility

[boesing]

Feature Request

Q	A
New Feature	yes
RFC	yes
BC Break	no

Summary

As of now, we are using some semver javascript library. Since that actually does not align with all the "features" of composer, we might want to create a little PHAR which uses composer/semver under the hood to simply exit with 0 if PHP version is supported by the constraint or 1 if not.

Adding some kind of child_process to our matrix javascript node binary and we can run that PHAR from within JS for a more accurate version detection.

This would also fix #86

Thoughts?

[boesing]

I've created https://github.com/boesing/composer-semver-docker
We can copy the executable from that container to /usr/local/bin so it can be used over the npm semver version.