introduce mrb_assert() in mruby.h
replace gc_assert with mrb_assert
replace assert with mrb_assert
if mrbc.run fail, abort rake
enabe MRB_DEBUG on travis_config.rb
Rename --pull_gems to --pull-gems
Merge pull request #1412 from h2so5/rename-pull-gems-option

Rename --pull_gems to --pull-gems
I correct the mistakes of grammer in README.md.
I correct the mistakes of grammer in README.md. 'update' word is an countable noun.
Merge pull request #1413 from suzukaze/correct-mistake-of-grammer-in-readme.md

I correct the mistakes of grammer in README.md.
Merge pull request #1414 from suzukaze/correct-mistake-of-grammer-in-readme.md-2

I correct the mistakes of grammer in README.md.
Merge pull request #1410 from Fleurer/introduce-mrb-assert

add assert check for TravisCI