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

Makes the life of packagers and everybody who wants to build this
easier.

Closes #25.

That's what the README suggests and - more importantly - the cmake
config file works with.
Also use angle brackets instead of quotation marks in two cases.

Allowing to build against a system installed copy.