Add 1.4 and nightly to CI

[raphaelsaavedra]

Related to #155

[codecov]

Codecov Report

Merging #156 into master will decrease coverage by 0.65%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
- Coverage   95.57%   94.91%   -0.66%     
==========================================
  Files          14       14              
  Lines         858      885      +27     
==========================================
+ Hits          820      840      +20     
- Misses         38       45       +7     

Impacted Files	Coverage Δ
src/StateSpaceModels.jl	63.63% <0.00%> (-14.15%)	⬇️
src/estimation.jl	88.13% <0.00%> (-2.94%)	⬇️
src/utils.jl	93.87% <0.00%> (-1.92%)	⬇️
src/univariate_kalman.jl	87.64% <0.00%> (-0.74%)	⬇️
src/prints.jl	100.00% <0.00%> (ø)
src/structures.jl	100.00% <0.00%> (ø)
src/sqrt_kalman.jl	100.00% <0.00%> (ø)
src/kalman.jl	97.64% <0.00%> (+0.08%)	⬆️
src/models.jl	98.11% <0.00%> (+0.24%)	⬆️

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