Reduce CI load by skipping Julia 1.1, 1.2, and 1.3

[odow]

I think it is fine for us to only test the LTS release and the most current minor release.

[blegat]

Missing a d at the end of Disable

[codecov]

Codecov Report

Merging #2269 into master will decrease coverage by 0.28%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2269      +/-   ##
==========================================
- Coverage   91.41%   91.12%   -0.29%     
==========================================
  Files          42       42              
  Lines        4193     4192       -1     
==========================================
- Hits         3833     3820      -13     
- Misses        360      372      +12     

Impacted Files	Coverage Δ
src/Containers/vectorized_product_iterator.jl	35.71% <0.00%> (-7.15%)	⬇️
src/mutable_arithmetics.jl	85.71% <0.00%> (-6.35%)	⬇️
src/Containers/SparseAxisArray.jl	74.64% <0.00%> (-4.52%)	⬇️
src/copy.jl	88.00% <0.00%> (-4.00%)	⬇️
src/Containers/DenseAxisArray.jl	85.29% <0.00%> (-1.97%)	⬇️
src/operators.jl	87.67% <0.00%> (-0.69%)	⬇️

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[odow]

It also seems like travis builds both the PR and the push.

Should we add:

branches:
  only:
    - master
    - /^release-.*$/