Add include_paths in dependency gems.
Add more info about "include_paths and GEM dependency".
Merge pull request #2381 from monaka/pr-add-include_paths-in-dependency-gems

Append include paths in dependency gems
reserve a register for a block parameter in a for statement body; ref #2375
Merge branch 'master' of github.com:mruby/mruby
rename mrb_atexit to mrb_state_atexit to clarify a func will be called at the end of state, not process; ref #2211