add RUBY_ENGINE_VERSION

RUBY_ENGINE_VERSION is equivalent to MRUBY_VERSION. It would be a
standard way to get the interpreter version (without a case expression).

It's also already defined by CRuby 2.3, JRuby 9000, Opal, and Rubinius.

src/version.c

@@ -4,8 +4,11 @@
 void
 mrb_init_version(mrb_state* mrb)
 {
+  mrb_value mruby_version = mrb_str_new_lit(mrb, MRUBY_VERSION);
+
   mrb_define_global_const(mrb, "RUBY_VERSION", mrb_str_new_lit(mrb, MRUBY_RUBY_VERSION));
   mrb_define_global_const(mrb, "RUBY_ENGINE", mrb_str_new_lit(mrb, MRUBY_RUBY_ENGINE));
+  mrb_define_global_const(mrb, "RUBY_ENGINE_VERSION", mruby_version);
   mrb_define_global_const(mrb, "MRUBY_VERSION", mrb_str_new_lit(mrb, MRUBY_VERSION));
   mrb_define_global_const(mrb, "MRUBY_RELEASE_NO", mrb_fixnum_value(MRUBY_RELEASE_NO));
   mrb_define_global_const(mrb, "MRUBY_RELEASE_DATE", mrb_str_new_lit(mrb, MRUBY_RELEASE_DATE));