Fix: add check for missing "mtimes"

[sshev]

watchpack 1.3.0 no longer returns mtimes and build fails
Fixes #378

[johnnyreilly]

Heya, you probably want to swap mtimes for getTimes() as in here: TypeStrong/ts-loader#482

For more details see here: https://blog.johnnyreilly.com/2017/02/under-duck-afternoon-in-open-source.html

[johnnyreilly]

mtimes will be removed in future

[s-panferov]

@johnnyreilly thanks