chore(package): up version to 3.1.0

refactor(formatters): set `alreadyWritten` flag in one place

chore(package): up version to 3.0.7

chore(test): change casing in `describe_Levels*` tests

chore(package): up version to 3.0.6

chore(package): up version to 3.0.5

chore(examples): restore packages in .NET Framework example

chore(examples): upgrade .NET Core example to NSpec.3.0.4-dev-27

chore(package): up version to 3.0.4

chore(package): up version to 3.0.3

Also, drop using AppVeyor private project feed