add [compat] to Project.toml

[bjarthur]

i've manually tested all versions of the dependencies which require at least julia 0.7 and specified which ones work with Compose. notably, DataStructures 0.9 and 0.10 did not.

[codecov-io]

Codecov Report

Merging #351 into master will decrease coverage by 0.85%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #351      +/-   ##
==========================================
- Coverage   39.75%   38.89%   -0.86%     
==========================================
  Files          18       18              
  Lines        3064     3147      +83     
==========================================
+ Hits         1218     1224       +6     
- Misses       1846     1923      +77

Impacted Files	Coverage Δ
src/list.jl	20.68% <0%> (-4.32%)	⬇️
src/container.jl	35.26% <0%> (-3.02%)	⬇️
src/fontfallback.jl	47.57% <0%> (-2.97%)	⬇️
src/cairo_backends.jl	53.23% <0%> (-2.12%)	⬇️
src/form.jl	50% <0%> (-1.15%)	⬇️
src/svg.jl	69.04% <0%> (-1.01%)	⬇️
src/table.jl	14.28% <0%> (-0.72%)	⬇️
src/Compose.jl	27.38% <0%> (-0.67%)	⬇️
src/batch.jl	9.58% <0%> (-0.56%)	⬇️
src/property.jl	21.47% <0%> (-0.14%)	⬇️
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ade8dbe...7cce74d. Read the comment docs.