Assertion check for webpack version

[1999]

It looks like awesome-typescript-loader@5 works with at least webpack version 4 but this is not checked in its code. So the errors that users get look like this:

TypeError: Cannot read property 'watchRun' of undefined
    at setupWatchRun (/Users/dsorin/ecosystem/dac/packages/app-management-apollo/node_modules/awesome-typescript-loader/src/instance.ts:372:16)
    at Object.ensureInstance (/Users/dsorin/ecosystem/dac/packages/app-management-apollo/node_modules/awesome-typescript-loader/src/instance.ts:141:2)
    at compiler (/Users/dsorin/ecosystem/dac/packages/app-management-apollo/node_modules/awesome-typescript-loader/src/index.ts:47:19)
    at Object.loader (/Users/dsorin/ecosystem/dac/packages/app-management-apollo/node_modules/awesome-typescript-loader/src/index.ts:16:12)
    at LOADER_EXECUTION (/Users/dsorin/ecosystem/dac/node_modules/loader-runner/lib/LoaderRunner.js:119:14)
    at runSyncOrAsync (/Users/dsorin/ecosystem/dac/node_modules/loader-runner/lib/LoaderRunner.js:120:4)
    at iterateNormalLoaders (/Users/dsorin/ecosystem/dac/node_modules/loader-runner/lib/LoaderRunner.js:229:2)
    at Array.<anonymous> (/Users/dsorin/ecosystem/dac/node_modules/loader-runner/lib/LoaderRunner.js:202:4)
    at Storage.finished (/Users/dsorin/ecosystem/dac/node_modules/enhanced-resolve/lib/CachedInputFileSystem.js:40:15)
    at /Users/dsorin/ecosystem/dac/node_modules/enhanced-resolve/lib/CachedInputFileSystem.js:77:9
    at /Users/dsorin/ecosystem/dac/node_modules/graceful-fs/graceful-fs.js:78:16
    at FSReqWrap.readFileAfterClose [as oncomplete] (fs.js:511:3) 'TypeError: Cannot read property \'watchRun\' of undefined\n    at setupWatchRun (/Users/dsorin/ecosystem/dac/packages/app-management-apollo/node_modules/awesome-typescript-loader/src/instance.ts:372:16)\n    at Object.ensureInstance (/Users/dsorin/ecosystem/dac/packages/app-management-apollo/node_modules/awesome-typescript-loader/src/instance.ts:141:2)\n    at compiler (/Users/dsorin/ecosystem/dac/packages/app-management-apollo/node_modules/awesome-typescript-loader/src/index.ts:47:19)\n    at Object.loader (/Users/dsorin/ecosystem/dac/packages/app-management-apollo/node_modules/awesome-typescript-loader/src/index.ts:16:12)\n    at LOADER_EXECUTION (/Users/dsorin/ecosystem/dac/node_modules/loader-runner/lib/LoaderRunner.

That's not helpful, especially when you're using React storybooks and you get webpack@2 by default 🤦‍♂️

[1999]

@s-panferov I'm pretty sure failing build doesn't relate to this small PR change 🤷‍♂️