Exception thrown windows machine with FIPS enabled

[marknadig]

When PusherServer is used on a machine with FIPS enabled, a runtime exception is thrown ": This implementation is not part of the Windows Platform FIPS validated cryptographic algorithms" in CryptoHelper.GetMd5Hash() due to MD5 not being validated as FIPS compliant.

That said, the intent is to just create a hash, not actually secure anything. A solution that would allow PusherServer to run on a FIPS enabled environment is to "roll your own Md5" which is pretty straightforward
https://stackoverflow.com/a/52166330/600299

Would you accept a PR for this?

[benw-pusher]

@marknadig just got back to you on your ticket before I saw this. We would be happy to review a PR for this to help overcome the FIPS compliance.