build: update ts-jest

[alexandru]

Trying to update ts-jest from 20.0.4 to 20.0.6, however the tests ends up failing with errors like:

 FAIL  test/core/either.test.ts
  ● Test suite failed to run

    ReferenceError: babelHelpers is not defined
      
      at src/core/errors.ts:24:37
      at Object.<anonymous> (src/core/errors.ts:47:114)
      at Object.<anonymous> (src/core/index.ts:25:10)
      at Object.<anonymous> (src/funfix.ts:24:10)
      at Object.<anonymous> (test/core/either.test.ts:20:16)
      at handle (node_modules/worker-farm/lib/child/index.js:41:8)
      at process.<anonymous> (node_modules/worker-farm/lib/child/index.js:47:3)
      at emitTwo (events.js:106:13)
      at process.emit (events.js:194:7)
      at process.nextTick (internal/child_process.js:766:12)
      at _combinedTickCallback (internal/process/next_tick.js:73:7)
      at process._tickCallback (internal/process/next_tick.js:104:9)

[codecov]

Codecov Report

Merging #8 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master     #8   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          11     11           
  Lines         449    449           
  Branches      150    150           
=====================================
  Hits          449    449

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