Temporary pangolayout fix for Julia 0.7 compatibility

[dehann]

See discussion here: GiovineItalia/Gadfly.jl#1206

cc @bjarthur , @d9w

Thanks!

[codecov-io]

Codecov Report

Merging #325 into master will decrease coverage by 48.16%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #325       +/-   ##
===========================================
- Coverage   79.44%   31.28%   -48.17%     
===========================================
  Files          14       18        +4     
  Lines        1192     3027     +1835     
===========================================
  Hits          947      947               
- Misses        245     2080     +1835

Impacted Files	Coverage Δ
src/pango.jl	0% <ø> (ø)
src/Compose.jl	10.12% <0%> (-89.88%)	⬇️
src/table.jl	9.52% <0%> (-84.6%)	⬇️
src/list.jl	19.23% <0%> (-80.77%)	⬇️
src/property.jl	20.25% <0%> (-79.75%)	⬇️
src/misc.jl	29.62% <0%> (-54.59%)	⬇️
src/form.jl	44.18% <0%> (-52.76%)	⬇️
src/measure.jl	24.76% <0%> (-51.71%)	⬇️
src/cairo_backends.jl	36.25% <0%> (-46.02%)	⬇️
src/container.jl	43.75% <0%> (-44.49%)	⬇️
... and 9 more

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