Fix undefined method rindex

[chatgris]

Hello,

This commit fix :

.bundle/gems/ruby/1.9.1/gems/rails_autolink-1.0.1/lib/rails_autolink.rb:141:in `auto_linked?'
.bundle/gems/ruby/1.9.1/gems/rails_autolink-1.0.1/lib/rails_autolink.rb:93:in `block in auto_link_urls'
.bundle/gems/ruby/1.9.1/gems/activesupport-3.1.0.rc2/lib/active_support/core_ext/string/output_safety.rb:125:in `gsub'
.bundle/gems/ruby/1.9.1/gems/activesupport-3.1.0.rc2/lib/active_support/core_ext/string/output_safety.rb:125:in `gsub'
.bundle/gems/ruby/1.9.1/gems/rails_autolink-1.0.1/lib/rails_autolink.rb:89:in `auto_link_urls'
.bundle/gems/ruby/1.9.1/gems/rails_autolink-1.0.1/lib/rails_autolink.rb:65:in `auto_link'

which appears in rails 3.1.0rc2.

Cheers

[pksunkara]

Pull this fast and please release a new minor version.

I have been getting lots of errors.

Thanks