FluentAssertions dependencies

The packages had a dependency on a very specific version of FluentAssertions, that caused issues when you were using the sink in a project that had a newer version of FluentAssertions than was expected by Serilog.Sinks.InMemory.Assertions.
So with this release you probably don't have to spend hours trying to figure out why you're getting a MissingMethodException when calling Should().HaveMessage("....")