fix: memory leak in TransformNormalModuleFactoryPlugin

[SevInf]

Problem: plugin wraps createLoaderContext without checking if it was
already done before. As a result, in a watch mode more and more wrappers
get added on top of the function and eat more and more memory. Fixed by
saving original function on module instance itself and wrapping saved
function instead.

Fixes #409

[SevInf]

@mzgoddard I'm pretty sure failing tests are just flaky. There were two builds on travis (first and second) and different tests fail. The only difference between those two builds is formatting.

[mzgoddard]

@SevInf thank you for finding this.

I'm increasing the timeout again in another PR for all tests since travis isn't reliable testing full webpack builds.