Update to the newest libuv version
Don't require busted.

Only provide the unittest target if busted was found.  And only build
nvim-test if the unittest target exists by excluding nvim-test from all.
Note: this means nvim-test won't be built by default, but it will be
built when you try to run unittests.
Merge branch 'master' into luaviml

Conflicts:
	src/CMakeLists.txt