Make Pkg.update() return nothing

[zhmz90]

When Pkg.update() finishes with no error, false is returned which can confuse users.

Current state is:

INFO: Updating OpenGene master... b28b0d93 → c3139d86
INFO: Computing changes...
INFO: Removing BufferedStreams v0.1.1
INFO: Removing Libz v0.1.1
false

julia> 

[tkelman]

should probably test this

[zhmz90]

Ok, I will make tests later.

[tkelman]

there are already a few calls to Pkg.update during test/pkg.jl you could just add checks to:

julia/test/pkg.jl

Lines 186 to 205 in e56c2a1

	Pkg.update()
	Pkg.installed()["Example"] == v"0.4.0"
	end
	# add a directory that is not a git repository
	begin
	mkdir(joinpath(Pkg.dir(), "NOTGIT"))
	Pkg.installed("NOTGIT") == typemin(VersionNumber)
	Pkg.installed()["NOTGIT"] == typemin(VersionNumber)
	end
	begin
	# don't bork when a Pkg repo is bare, issue #13804
	pth = joinpath(Pkg.dir(), "BAREGIT")
	mkdir(pth)
	# create a bare repo (isbare = true)
	repo = LibGit2.init(pth, true)
	@test repo.ptr != C_NULL
	finalize(repo)
	Pkg.update()

[zhmz90]

@tkelman Thanks for your helpful instructions, I have made a new PR now.