Use Spark 2.4.5 in Travis and Github Actions

[HyukjinKwon]

Spark 2.4.5 was released. This PR proposes to use it in both Travis CI and Github Actions.

[codecov-io]

Codecov Report

Merging #1269 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1269   +/-   ##
=======================================
  Coverage   95.16%   95.16%           
=======================================
  Files          35       35           
  Lines        7151     7151           
=======================================
  Hits         6805     6805           
  Misses        346      346

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[itholic]

LGTM

[HyukjinKwon]

Let me just merge this cc @ueshin