deps: bump leveldown version for iojs compat

[yoshuawuyts]

This bumps the leveldown version to ensure io.js compatibility. Might warrant a major version bump since leveldown also went post 1.0.0. Thanks!

Ps. By merging this you help in bringing clocker to io.js!

[rvagg]

this has to wait for levelup@1.0.0 unfortunately

the interim solution is to backport io.js support to leveldown@0.10 which is a WIP (there's a PR for this)

[yoshuawuyts]

Ah, I see. Adding links for completion:

• leveldown 0.10 backport
• levelup 1.0.0 PR -- which unfortunately looks a bit stale :(

[rvagg]

levelup 1.0.0 has become our (my?) node 0.12 ... we need to cut cut cut and ship, I just need to find a few free hours to make it all happen but that's kind of tricky right now (would you rather me put my limited spare time in to io.js or level?)

[yoshuawuyts]

Oh, I didn't mean to sound disgruntled, apologies if it appeared like that! Really appreciate what you've done with the place, hah. The 0.10 backport looks promising. Closing this PR in favor of a new one once the backport is released.