Use SnakeYAML version for Psych::LIBYAML_VERSION.

There has been some confusion about whether JRuby is affected by
recent libyaml vulnerabilities. This confusion was compounded by
our reporting libyaml version in Psych's constants when we don't
actually use libyaml.

This change modifies the libyaml version constants to report the
current SnakeYAML version. This may cause confusion of a different
sort, since JRuby claims a libyaml version that does not exist,
but since this is our "yaml lib" it's not too far of a stretch.