Declarations update independent of compiler.watchFileSystem

Uses watching.compiler.fileTimestamps which is set directly in
the compiler. See https://github.com/webpack/webpack/blob/master/lib/Compiler.js#L105.

Fixes the #155
when using the webpack.OldNodeWatchFileSystem.

index.ts

@@ -488,7 +488,8 @@ function ensureTypeScriptInstance(loaderOptions: LoaderOptions, loader: any): {
 
     // manually update changed declaration files
     loader._compiler.plugin("watch-run", (watching, cb) => {
-        var mtimes = watching.compiler.watchFileSystem.watcher.mtimes;
+        var mtimes = watching.compiler.fileTimestamps ||
+                     watching.compiler.watchFileSystem.watcher.mtimes;
         Object.keys(mtimes)
             .filter(filePath => !!filePath.match(/\.d\.ts$/))
             .forEach(filePath => {