Invert the meaning of the "dev" option to correct value

The 1.0 style dev flag is incorrectly designed.  Any time a boolean
means "yes, do the thing", that should be the "true" option. In this
case, the flag is "dev", so "dev:true" should mean, "Yes, include dev
dependencies".

However, it was set as "dev:true" meaning "Yes, don't include the dev
dependencies", which is an awkward "no means yes" kind of situation.

I think that @davglass was trying to do the prudent thing and disrupt
the current behavior as little as possible, so that a default "falsey"
choice would do what it currently does, and not treat devDependencies as
extraneous.  I had misread his original pull request, and approved it.

It's unfortunate that this means yet another breaking change.
Thankfully, that's what the Major version in SemVer is for, and not many
libraries use this, apart from npm itself :)