Fix various Julia v0.7 warnings

[blegat]

No description provided.

[codecov]

Codecov Report

Merging #1349 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1349   +/-   ##
=======================================
  Coverage   89.33%   89.33%           
=======================================
  Files          24       24           
  Lines        3386     3386           
=======================================
  Hits         3025     3025           
  Misses        361      361

Impacted Files	Coverage Δ
src/macros.jl	88.48% <100%> (ø)	⬆️
src/Derivatives/coloring.jl	97.68% <100%> (ø)	⬆️
src/Derivatives/subexpressions.jl	100% <100%> (ø)	⬆️
src/Derivatives/linearity.jl	95.91% <100%> (ø)	⬆️
src/Derivatives/conversion.jl	98.11% <100%> (ø)	⬆️
src/JuMPArray.jl	76.04% <100%> (ø)	⬆️
src/Derivatives/topological_sort.jl	100% <100%> (ø)	⬆️
src/Derivatives/types.jl	100% <100%> (ø)	⬆️
src/variables.jl	88.55% <100%> (ø)	⬆️

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