Unfortunate peer dependency on babel when unit testing

[sam]

Looks like @aws-cdk/assert requires babel-jest? Unit tests of the examples package (and therefore any consumer of punchcard) break without the following devDependencies:

"@babel/core": "^7.4.5",
"babel-core": "^7.0.0-bridge.0",

See #38

[sam]

Also probably related to the janky way we use webpack to bundle lambdas: #35