Add Travis CI job to test JMeter with IBM s390x

[namrata-ibm]

Motivation and Context

As Travis CI officially supports s390x builds, adding support for same.

How Has This Been Tested?

Travis job passes

Screenshots (if appropriate):

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly.

[codecov-io]

Codecov Report

Merging #550 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #550      +/-   ##
============================================
+ Coverage     56.11%   56.11%   +<.01%     
- Complexity    10020    10022       +2     
============================================
  Files          1024     1024              
  Lines         62911    62911              
  Branches       7063     7063              
============================================
+ Hits          35301    35304       +3     
+ Misses        25132    25131       -1     
+ Partials       2478     2476       -2

Impacted Files	Coverage Δ	Complexity Δ
...n/java/org/apache/jmeter/reporters/Summariser.java	85.49% <0%> (+0.76%)	18% <0%> (+1%)	⬆️
...a/org/apache/jmeter/timers/PoissonRandomTimer.java	78.37% <0%> (+5.4%)	10% <0%> (+1%)	⬆️

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[vlsi]

@namrata-ibm , thanks for the notice and for the PR