Remove Julia 1.3 from Travis CI configuration

[notZaki]

Julia 1.3 doesn't need to be in the tests.
It was added when Julia 1.3 was the latest version and I didn't know that the version '1' would always use the latest release.

[codecov]

Codecov Report

Merging #58 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #58   +/-   ##
=======================================
  Coverage   87.82%   87.82%           
=======================================
  Files           1        1           
  Lines         312      312           
=======================================
  Hits          274      274           
  Misses         38       38           

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[notZaki]

A more aggressive approach could be to remove the latest release, i.e. '1', from the tests as well. In theory, the builds on 1.0 and nightly should encompass everything in between.

[DilumAluthge]

A more aggressive approach could be to remove the latest release, i.e. '1', from the tests as well. In theory, the builds on 1.0 and nightly should encompass everything in between.

In theory.... sometimes nightly does weird stuff though.

[DilumAluthge]

One thing we could consider would be switching from Travis to GitHub Actions. Usually GH Actions is much faster.

If you're interested, let me know - I'm happy to make a PR to set up GH Actions for CI testing on this repo.

[notZaki]

That sounds great!
I'm curious about GH Actions too, so seeing it applied in a setup I'm previously familiar with would be a useful learning opportunity for me.