fix: move babel-jest to development dependencies

[hvolschenk]

move babel-jest to devDependencies as it is not required when installed update babel-jest and jest to 24.1.0 to remove npm audit vulnerabilities

Summary

Fixes #32

Requirements (place an x in each [ ])

• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [ ] I've written tests to cover the new code and functionality included in this PR.
• [x] I've read, agree to, and signed the Contributor License Agreement (CLA).

Other

I see there are a few other dependencies that are also out of date, but tried to rather keep this PR focused to this one issue. Please let me know if you'd rather have me also update the other dependencies.

[CLAassistant]

All committers have signed the CLA.

[codecov[bot]]

Codecov Report

Merging #33 into master will not change coverage. The diff coverage is n/a.

@@          Coverage Diff           @@
##           master     #33   +/-   ##
======================================
  Coverage    93.8%   93.8%           
======================================
  Files           2       2           
  Lines         113     113           
  Branches       21      21           
======================================
  Hits          106     106           
  Misses          6       6           
  Partials        1       1

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[AnujRNair]

Fixed in https://github.com/slackhq/csp-html-webpack-plugin/pull/34