(maint) Don't try to invoke a constant as a method

While methods on constants can be invoked with the scope resolution
operator, EG `SemVer::valid?`, the reverse isn't true for retrieving
constants. This fixes the lookup of the SemVer::MIN constant.
Merge pull request #1685 from adrienthebo/maint-constant_lookup

(maint) Don't try to invoke a constant as a method