use C style comments instead of C++ style comments

According to CONTRIBUTING.md,

Don't use C++ style comments

 /* This is the prefered comment style */

Use C++ style comments only for temporary comment e.g. commenting out some code lines.
Merge branch 'issues/comment_style' of https://github.com/cubicdaiya/mruby into cubicdaiya-issues/comment_style
revert changes to mrbconf.h
Merge branch 'master' of github.com:mruby/mruby