do not load paths that are missing

this will allow users to use newer rdoc with ri.vim
Merge pull request #13 from korin/rdoc-4.0.1

do not load paths that are missing
rdoc gem version require

I don't known why but `gem 'rdoc', '=> 3.8'` stopped working for me on `ruby-2.0.0-p247`. I change it to `gem 'rdoc', '>=3.8'` and it work fine now.
Merge pull request #15 from korin/patch-1

rdoc gem version require
Fix for #20
remove outdated keymap to help()
Fix for #22; not sure of side effects
Fixing the logic of rising exceptions when iterating through the stores
Merge pull request #24 from v-yadli/master

Fixing the logic of rising exceptions when iterating through the stores
Add Matthias as principal maintainer #23
Cleanup of ri.vim, fixes #16
README updates, fixes #12
Dead code removal
Get rid of digest in brackets, fixes #25