Improve `rbenv init` instructions for fish

Fish user config file `~/.config/fish/config.fish` loads for every
instance of fish shell, not just interactive ones. Since it's
unnecessary and dangerous to eval `rbenv init -` output in
non-interactive shells, wrap the invocation in a conditional that checks
if the current shell is interactive.

Fixes #501
Merge remote-tracking branch 'sstephenson/master'

* sstephenson/master:
  Improve `rbenv init` instructions for fish