Recommend setting enable_reloading on newer Rails versions

[nvasilevski]

README recommends to set newer enable_reloading config

spring/README.md

Line 74 in b6dc87e

Ensure that `config.enable_reloading` is `true` in the environments that

while the raised message still asks to tweak old cache_classes config which may eventually be removed.

Let's update the error to ask applications to configure enable_reloading where available.

Does checking for respond_to? seems reasonable? Any reason to check Rails version instead?