Support Rails 4.0.0

Rails 4.0.0 removes rails_env task. We can just remove it.

This change requires Rails 4.0.0. If you want to use ActiveGroonga
with Rails < 4, use older version.

GitHub: fix #9

Reported by Eito Katagiri. Thanks!!!
travis: use public mailing list
travis: use 2.0.0 instead of 1.8.7
Use " (double quote) for string literal
Use " (double quote) for string literal
Remove needless require
Use " (double quote) for string literal
Bump version to use the same version as Rails
Add 4.0.0 entry
Update po

    % rake reference:po:update
doc ja: translate
Bump version

4.0.0 ->
4.0.1
    ^
Update site directory