babel-jest is missing

[niahmiah]

tests do not work correctly out of the box, due to the import error mentioned here: jestjs/jest#2081

installing babel-jest as suggested in that thread fixes this issue.

[niahmiah]

Also need to bump jest version to match. Jest 23 is recommended for webpack 4 anyway.

[calvinl]

@niahmiah Thanks for the reports. I'll take a look at these shortly, however I'm always willing to accept a PR. :)