needs update for string.data changes

[stevengj]

After JuliaLang/julia#19449, soon to be merged for Julia 0.6, you will no longer be able to access the raw bytes of a string via string.data; instead, do Vector{UInt8}(string). For example, this affects:

https://github.com/quinnj/WeakRefStrings.jl/blob/34d5207e0c3e0c4b133ccc8d226a9e509e530df1/src/WeakRefStrings.jl#L64

By the way, length(x) in that function looks wrong. I think you want sizeof(x), the number of bytes, not the length (the number of codepoints).