include mruby/version.h in mruby.h

To write a mrbgem can be used with both master and 1.0.0 version, I tried to use `MRUBY_RELEASE_NO` but it is defined in `version.h` and the file is missing in 1.0.0 so the line `#include <mruby/version.h>` causes a compilation error.  With including `version.h` in `mruby.h`, we can write conditionals as in https://github.com/mruby/mruby/commit/95e2be6508257af3ed4989b39c02d281ae6c5330#commitcomment-5305246 .