More reusable rvm_prompt_info in lib/rvm.zsh

By removing the wrapping parens this function can be reused and the output more easily customized by the consumer.

For example, in my prompt, want to combine my RVM + Git info like:

machine-name: ~/path/here [ruby-1.9.2-p290@my_project | master]

However, the existing wrapping parenthesis force me to either roll my own, or strip the parens before outputting the info to the prompt.