drop support for julia 0.7

[bjarthur]

fixes #1328

[codecov-io]

Codecov Report

Merging #1332 into master will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1332      +/-   ##
==========================================
- Coverage   90.07%   90.02%   -0.06%     
==========================================
  Files          36       36              
  Lines        3941     3941              
==========================================
- Hits         3550     3548       -2     
- Misses        391      393       +2

Impacted Files	Coverage Δ
src/theme.jl	69.49% <0%> (-1.7%)	⬇️
src/mapping.jl	84.46% <0%> (-0.98%)	⬇️

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