reduce the init size of stack
bug fixed
Implement default gem info to use when there is no depending gem is defined.
Merge remote-tracking branch 'remotes/mruby/master' into enhance
fixed.
Merge pull request #2109 from mirichi/enhance

reduce the init area of stack
Merge pull request #2108 from take-cheeze/default_gem_info

Implement default gem info to use when there is no depending gem loaded.
Fix MSVC warning for gc.c
Merge pull request #2111 from kyab/fix_msvc_warning_gc.c

Fix MSVC warning for gc.c