add RGSS_VERSION (RGSS3)

Even though 3.0.1 is the current version, 3.0.0 is used, because mkxp doesn't
implement the changes like not raising a TypeError for 'Color.new == nil' yet.
3.0.1 (finally) returns false. Other changes don't affect mkxp (I think).

See <http://tkool.jp/support/download/rpgvxace/rpgvxace_update> (Japanese).
Merge pull request #61 from cremno/mri-add-rgss_version

add RGSS_VERSION (RGSS3)