Check that purs version is at least what the package-set specifies

[f-f]

As detailed in #98 (comment), we should run purs --version every time we want to install packages, to check that we're running at least the compiler version specified by the package-set (see spacchetti/spacchetti#63)

If the version isn't there we should skip the check (see #94 for when we could fail instead), if the version is there we should fail if it's not SemVer-compatible (e.g. 0.11.x is not compatible with 0.12.x, but 0.12.x is compatible with 0.12.y, if x > y and the compiler has x), so that the user can install the right version of the compiler.