backslash escapes in regexp should be addressed by Regexp processor; close #2005
Refactor MRuby::Gem::List#check.
* Create mrbgem table before dependencies check.
* Topoligical sort gem list.
* Check circular mrbgem dependency.
Merge pull request #2006 from take-cheeze/tsort_dep

Refactor MRuby::Gem::List#check.