Use ruby -w by default

[hgmnz]

No description provided.

[spastorino]

/cc @drbrain @jimweirich

[jimweirich]

Won't change this in a minor release. Will consider it for the next major release.

[jimweirich]

Although I'm in favor of this change in principle, I've decided not to include it in the 0.9.3/10.0.0 changes. If I dropped it in without warning, every single rails application ever will start spewing errors.

We need to work out a more gentle introduction of this change (e.g. explore deprecation options, get the rails guys to explicitly set the warning option, etc.).