Add 'module' entry to allow Webpack to tree shake it

[felixmosh]

Bundlers such as Webpack are not support jsnext:main entry, but do support module.

I've just added that entry, I've checked on my build, and webpack was able to enable module concat optimisation and tree shaking.

[felixmosh]

someone can assist, how the build fails on webpack configuration? :
It passes locally

[christopherthielen]

When we run tests on travis, we run unit tests AND downstream integration tests (npm run test:downstream). Don't worry about fixing those, I'll check on them. Thanks for the PR!

Note: I do recall there was a reason I used jsnext:main over module and I think it had something to do with Angular AoT compilation. However, Angular has had a few major release since then, and we've done some other workaround to hint to Angular how to consume the library. If the downstream tests pass, then I'm fairly confident this change won't affect Angular consumers.