Adapt to changes in YARA v4.0 API (#141)

* Upgrade YARA submodule and adapt to new API.

* Upgrade YARA and adapt to API changes.

* Fix build in Appveyor (#131)

* The precompiled OpenSSL is now extracted from the NuGet package generated by https://ci.appveyor.com/project/plusvic/openssl.

* OpenSSL was upgraded to version 1.1.1.