Update spark-core_2.11 to 2.3.1.

[ruebot]

GitHub issue(s): n/q

What does this Pull Request do?

Updates spark version to 2.3.1 to address the following vulnerabilities:

• CVE-2018-8024 https://nvd.nist.gov/vuln/detail/CVE-2018-8024
• CVE-2018-1334 https://nvd.nist.gov/vuln/detail/CVE-2018-1334
• CVE-2018-17190 https://nvd.nist.gov/vuln/detail/CVE-2018-17190
• CVE-2018-11770 https://nvd.nist.gov/vuln/detail/CVE-2018-11770

How should this be tested?

Test suite, and TravisCI should take care of it.

[codecov-io]

Codecov Report

Merging #312 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #312   +/-   ##
=======================================
  Coverage   75.84%   75.84%           
=======================================
  Files          41       41           
  Lines        1151     1151           
  Branches      202      202           
=======================================
  Hits          873      873           
  Misses        209      209           
  Partials       69       69

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[ianmilligan1]

TravisCI turned green, and is building nicely locally.

[ruebot]

@lintool want to merge this one? Should be good to go.