Change refs for joyent/libuv repo to libuv/libuv

* Replace references from github.com/joyent/libuv to
github.com/libuv/libuv

Similar to https://github.com/neovim/neovim/pull/1562
Merge pull request #6 from Happy-Dude/change-joyent-libuv-refs

Change refs for joyent/libuv repo to libuv/libuv