simplify string equality check
Hash#replace should copy default as well; close #2004
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.
Update mruby-head installed on Travis CI

This way other projects can test against it.
Merge pull request #2008 from rkh/patch-1

Update mruby-head installed on Travis CI