Brew Install Rtorrent not Working
I am facing the same issue. I believe it is caused by the newly added gcc v5.1.0 package.

Rolling back to gcc v4.9.2 solved the installation issue with rtorrent v0.9.4 package. You can rollback to gcc v4.9.2 two different methods based on the current state of your homebrew installation. 

Method A

If you have not performed brew cleanup after brew upgrade --all then you can simply switch to older version of gcc by running brew switch gcc 4.9.2. For further information read this answer.

Method B

Unfortunately if you have a habit of running brew cleanup after brew upgrade --all (like me ;-(), follow the steps mentioned below to install gcc v4.9.2 package.

brew uninstall gcc
brew tap homebrew/versions
brew install gcc49


Final Step

Install rtorrent by running the below mentioned command in your teminal.

brew install rtorrent