Migrate from ts-jest to babel typescript transformer

[mohanraj-r]

From #2

Pros

• With Jest 24 there's much better support for Typescript
• The maintainer of ts-jest has basically said to just use native Jest: What does the new major release of Jest (24) means for this library kulshekhar/ts-jest#961 (comment)
• worth considering for the future since other projects have reported a speed improvement moving away from ts-jest.
• Babel is already a dependency of Jest so technically it's already pulled it in to the project as a transitive dependency.

caveats

• Eval the limitations and tradeoffs
  • some features might not be available in babel yet
  • caveats
• Tried replacing ts-jest with babel/preset-typescript as per https://jestjs.io/docs/en/getting-started. Ran into some errors with jest trying to run the .js files as well as .d.ts files in addition to .ts files. Probably needs more config than what is mentioned in those docs.