update utf8proc -> libmojibake

[stevengj]

Now that we have our own fork shaping up rapidly, it will make sense soon to switch Julia over to it.

This will fix #7582, now that @jiahao has updated libmojibake for Unicode 7 (JuliaStrings/utf8proc#9).

We should probably make a decision on JuliaStrings/utf8proc#10 first.

[stevengj]

(GNU libunistring, which is LGPL, also looks like a sane option. But it is about 100x more source code than utf8proc/libmojibake, and does a bunch of things that we implement more cleanly on the Julia side.)

[juliohm]

@stevengj, the migration will actually occur? What is the current status?

[stevengj]

@juliohm, we are planning on the migration, but I was hoping to have a test suite for libmojibake (JuliaStrings/utf8proc#13) before we switch.