Fix CI issues w/ mtxclient.  Thanks to @MonokelPinguin 

Current AppVeyor image includes a very old vcpkg snapshot. Because of
that, this doesn't include https://github.com/Microsoft/vcpkg/pull/5258
specifically
https://github.com/Microsoft/vcpkg/pull/5258/commits/c895bb658d52bee471169460c711fdd90e3d5b7b

Because of that, find_package only works for find_package(nlohmann-json)
but not find_package(nlohmann_json). Updating the ports cache should fix
that.

Add missing iostream include.  Should hopefully fix #11 .  Thanks @MonokelPinguin 

Otherwise build breaks with boost>=1.70

Should fix #11

Workaround synapse bug #4898 Thanks @MonokelPinguin 

Ignore type_error while parsing timestamp of read receipts, because
Synapse sends out data for them, when using workers.

I think I finally worked out the issues w/ linking against the
nlohmann JSON library.  I have updated the build and the various
CI scripts.

Fix Findsodium.cmake version test. (thanks @jevolk )

This at least partially addresses #3.  I have a feeling that
additional updates will be needed at some point, but this
fixes the issue where mtxclient would throw an exception for
unrecognized event id formats, which caused nheko to crash.

Nlohman build improvements

This hopefully fixes the issue related to reading a file that travis
doesn't have read access to causing docker build to fail

This should address comments in Nheko-Reborn#10