Segment fault when use PyCall in v1.0

Julia version:

```
julia> versioninfo()
Julia Version 1.0.0
Commit 5d4eaca0c9 (2018-08-08 20:58 UTC)
Platform Info:
OS: macOS (x86_64-apple-darwin17.7.0)
CPU: Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz
WORD_SIZE: 64
LIBM: libimf
LLVM: libLLVM-6.0.0 (ORCJIT, skylake)
```

PyCall status. (not only master branch, but also v1.17.1)
```
(v1.0) pkg> status
Status `~/.julia/environments/v1.0/Project.toml`
[7876af07] Example v0.5.1
[28b8d3ca] GR v0.32.2
[7073ff75] IJulia v1.9.2+ [`~/.julia/dev/IJulia`]
[c601a237] Interact v0.8.0
[72cad168] LuxurySparse v0.1.0 [`~/.julia/dev/LuxurySparse`]
[91a5bcdd] Plots v0.18.0
[438e738f] PyCall v1.17.1+ [`~/.julia/dev/PyCall`]
```

This works fine on v0.7.0 however.

```
julia> using PyCall

julia> @pyimport numpy as np

signal (11): Segmentation fault: 11
in expression starting at no file:0
PyUnicode_InternInPlace at /Users/roger/.julia/packages/Conda/a196/deps/usr/lib/libpython3.6m.dylib (unknown line)
PyImport_Import at /Users/roger/.julia/packages/Conda/a196/deps/usr/lib/libpython3.6m.dylib (unknown line)
PyImport_ImportModule at /Users/roger/.julia/packages/Conda/a196/deps/usr/lib/libpython3.6m.dylib (unknown line)
_pyimport at /Users/roger/.julia/packages/PyCall/fiJ3/src/PyCall.jl:398
pyimport at /Users/roger/.julia/packages/PyCall/fiJ3/src/PyCall.jl:422
jl_fptr_trampoline at /Users/roger/Documents/Repos/julia/src/gf.c:1829
do_call at /Users/roger/Documents/Repos/julia/src/interpreter.c:324
eval_stmt_value at /Users/roger/Documents/Repos/julia/src/interpreter.c:363 [inlined]
eval_body at /Users/roger/Documents/Repos/julia/src/interpreter.c:682
jl_interpret_toplevel_thunk_callback at /Users/roger/Documents/Repos/julia/src/interpreter.c:799
unknown function (ip: 0xfffffffffffffffe)
unknown function (ip: 0x1108f35cf)
unknown function (ip: 0x3)
jl_interpret_toplevel_thunk at /Users/roger/Documents/Repos/julia/src/interpreter.c:808
jl_toplevel_eval_flex at /Users/roger/Documents/Repos/julia/src/toplevel.c:787
jl_toplevel_eval_in at /Users/roger/Documents/Repos/julia/src/builtins.c:622
eval at ./boot.jl:319
eval_user_input at /Users/roger/Documents/Repos/julia/usr/share/julia/stdlib/v1.0/REPL/src/REPL.jl:85
macro expansion at /Users/roger/Documents/Repos/julia/usr/share/julia/stdlib/v1.0/REPL/src/REPL.jl:117 [inlined]
#28 at ./task.jl:259
jl_apply at /Users/roger/Documents/Repos/julia/src/./julia.h:1536 [inlined]
start_task at /Users/roger/Documents/Repos/julia/src/task.c:268
Allocations: 49221215 (Pool: 49208124; Big: 13091); GC: 109
[1] 99686 segmentation fault julia --color=yes
```

Reproduce the crash with Julia 1.0, PyCall v1.17.1, and `@pyimport numpy as np` on macOS. Start in `src/PyCall.jl` around lines 398 and 422, using the reported stack trace to follow the import path. Done means the same import no longer causes a segmentation fault.

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