Make GitFlowVersion tests play nice when being run by a TeamCity agent

#32 introduced some test coverage in order to ensure that GFV was behaving as expected when being leveraged in a build tool chain on a TeamCity agent.

The way GFV detects this context is by probing the existence of a `TEAMCITY_VERSION` environment variable.

However, when those tests actually ran on a TeamCity agent, this very variable was always set which led to some failures.

This PR introduces a switch regarding the probing mechanism.
 - `TEAMCITY_VERSION` will be used when GFV is being run as a MSBuild task
 - `GitFlowVersion.Fake.TEAMCITY_VERSION` will be used when testing GFV