Composer package PHP-Parser is not stable

[lenton]

When installing via composer, it currently complains that the package nikic/PHP-Parser is not stable. This forces you to set your minimum-stabilty to dev which is not good.

This was attempted to be fixed here, but it now looks to be outdated: #35.

It's likely that we'll want to upgrade this package to version 1.1, as suggested by @Trismegiste in the PR.