chore: remove greenkeeper ignore for find-up

[realityking]

Originally this was added in #402 but find-up has since been updated

[realityking]

Also looks like greenkeeper has been disabled - all the more reason to remove it.

[JaKXz]

My opinion is that greenkeeper might become useful again, so this config isn't hurting us. My understanding is that we've disabled greenkeeper temporarily to try to reduce the issue/PR noise while we triage everything and try to get more issues closed/bugs fixed/PRs merged.

[realityking]

@JaKXz I agree but since find-up has already been updated ignoring it in the Greenkeeper config is actually harmful if you decide to use Greenkeeper again.

[bcoe]

@realityking @JaKXz I'm fine with removing this config -- I believe it's an artifact from wanting to be backwards compatible to Node 0.10.