Feature/respect ruby version file

1. Running `cap staging rvm:check` now always outputs something, not only when `log_level` is set to `debug`. It's now only automatically invoked on normal deploying if the `log_level` is `debug`, so no change in behavior here.

2. `cap staging rvm:check` is now executed in the current release path, so RVM will pickup any `.ruby-version` file and only fall back to the system defaults.

3. `rvm_ruby_version` is no longer set to `default`. Instead the ruby version reported by `cap staging rvm:check` is used by default now.