[LIVY-603][BUILD] Upgrade build spark version to 2.4.3

[yiheng]

What changes were proposed in this pull request?

Bump Spark 2.4 minor version to 2.4.3.

How was this patch tested?

Existing test

[codecov-io]

Codecov Report

Merging #179 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #179      +/-   ##
============================================
+ Coverage      68.6%   68.63%   +0.03%     
- Complexity      905      907       +2     
============================================
  Files           100      100              
  Lines          5666     5666              
  Branches        850      850              
============================================
+ Hits           3887     3889       +2     
  Misses         1225     1225              
+ Partials        554      552       -2

Impacted Files	Coverage Δ	Complexity Δ
...cala/org/apache/livy/scalaapi/ScalaJobHandle.scala	50% <0%> (-2.95%)	7% <0%> (ø)
...ain/java/org/apache/livy/rsc/driver/RSCDriver.java	78.81% <0%> (-0.43%)	42% <0%> (ø)
...main/scala/org/apache/livy/server/LivyServer.scala	35.96% <0%> (+0.49%)	11% <0%> (ø)	⬇️
.../scala/org/apache/livy/sessions/SessionState.scala	61.11% <0%> (+2.77%)	2% <0%> (ø)	⬇️
...in/java/org/apache/livy/rsc/driver/JobWrapper.java	88.57% <0%> (+5.71%)	9% <0%> (+1%)	⬆️

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[jerryshao]

Please change the PR title to add module name, like [LIVY-xxx][BUILD]?

[yiheng]

done

[jerryshao]

LGTM.

[jerryshao]

Thanks for the fix, merging to master branch.