Update requirements

[lostella]

Addresses #33:

• Add upper bounds to requirements
• Bump Julia requirement to 1.4 because of hyphen version specifiers
• Bump package version to 0.3.0
• Update test workflow to run on Julia 1.4 and 1.6

[codecov-commenter]

Codecov Report

Merging #34 (aa41e76) into master (6ada307) will increase coverage by 1.46%.
The diff coverage is n/a.

❗ Current head aa41e76 differs from pull request most recent head 86304e9. Consider uploading reports for the commit 86304e9 to get more accurate results

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
+ Coverage   90.46%   91.92%   +1.46%     
==========================================
  Files          17       17              
  Lines         430      483      +53     
==========================================
+ Hits          389      444      +55     
+ Misses         41       39       -2     

Impacted Files	Coverage Δ
src/syntax/problem.jl	100.00% <0.00%> (ø)
src/solvers/terms_extract.jl	100.00% <0.00%> (ø)
src/solvers/terms_properties.jl	100.00% <0.00%> (ø)
src/syntax/expressions/expression.jl	100.00% <0.00%> (ø)
src/syntax/expressions/multiplication.jl	100.00% <0.00%> (ø)
src/syntax/expressions/abstractOperator_bind.jl	100.00% <0.00%> (ø)
src/solvers/build_solve.jl	95.45% <0.00%> (+0.45%)	⬆️
src/solvers/minimize.jl	94.73% <0.00%> (+0.98%)	⬆️
src/calculus/precomposeNonlinear.jl	88.46% <0.00%> (+1.50%)	⬆️
src/syntax/expressions/addition.jl	81.60% <0.00%> (+2.12%)	⬆️
... and 4 more

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