Upgrade libuv to fix a leak on OSX

I haven't landed this fix upstream just yet, but it's opened as
joyent/libuv#1048. For now, I've locally merged it into my fork, and I've
upgraded our repo to point to the new revision.

Closes #11027
auto merge of #11038 : alexcrichton/rust/fix-osx-leak, r=cmr

I haven't landed this fix upstream just yet, but it's opened as
joyent/libuv#1048. For now, I've locally merged it into my fork, and I've
upgraded our repo to point to the new revision.

Closes #11027