WIP: Port to Julia v0.7

[epatters]

Do not merge. This PR is a start towards porting the library to Julia v0.7/v1.0. It's not working yet.

I get a segfault when trying to create a simple graph. I can't go down this rabbit hole right now but perhaps someone more knowledgeable can help move this forward.

julia> Graph("digraph G { n1 -> n2; }")
Process 32584 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=2, address=0x11b858000)
    frame #0: 0x000000011b858000
->  0x11b858000: xlatb
    0x11b858001: addb   %al, (%rax)
    0x11b858003: addb   %al, (%rax,%rax)
    0x11b858006: addb   %al, (%rax)
Target 0: (julia) stopped.
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=2, address=0x11b858000)
  * frame #0: 0x000000011b858000
    frame #1: 0x000000011bf22f81 libcgraph.dylib`aaglex + 1232
    frame #2: 0x000000011bf1f40e libcgraph.dylib`aagparse + 154
    frame #3: 0x000000011bf202a3 libcgraph.dylib`agconcat + 67
    frame #4: 0x000000011b767766
    frame #5: 0x00000001002b3a78 libjulia.dylib`do_call + 200
    frame #6: 0x00000001002b253c libjulia.dylib`eval_body + 1228
    frame #7: 0x00000001002b2a88 libjulia.dylib`jl_interpret_toplevel_thunk_callback + 408
    frame #8: 0x000000010010657c libjulia.dylib`enter_interpreter_frame + 28
    frame #9: 0x00000001002b2ae0 libjulia.dylib`jl_interpret_toplevel_thunk + 32
    frame #10: 0x0000000100120749 libjulia.dylib`jl_toplevel_eval_flex + 1993
    frame #11: 0x0000000100100882 libjulia.dylib`jl_toplevel_eval_in + 530
    frame #12: 0x0000000104eaa542 sys.dylib`japi1_eval_3982 at boot.jl:319 [opt]
    frame #13: 0x00000001055161c6 sys.dylib`japi1_eval_user_input_9798 at REPL.jl:85 [opt]
    frame #14: 0x0000000105516414 sys.dylib`japi1_#28_9796 [inlined] macro expansion at REPL.jl:117 [opt]
    frame #15: 0x00000001055163f0 sys.dylib`japi1_#28_9796 at task.jl:262 [opt]
    frame #16: 0x000000010010ab70 libjulia.dylib`start_task + 352