Add install of mail gem version 2.5.4

Perhaps it's my old problem of giving a question with the answer.  I doubt this is better but maybe it would be better to just say it? 

> You will need to install the mail gem if you want to use the _mailer_ handler.  If you have installed sensu from the packages you can install it like.

```shell
/opt/sensu/embedded/bin/gem install mail --no-ri --no-rdoc -v 2.5.4
```