getpwuid should not fail the build

[mcollins123]

Currently if you use env.CreateVersion to create a version file and don't have any of the following environment variables set: 'LOGNAME', 'USER', 'LNAME', 'USERNAME' and pwd.getpwuid(os.getuid())[0] fails then cuppa throws the exception from getpwuid failing the build.

Cuppa should probably catch this exception and set the user id to something like "?" or "Unknown". We hit this issue when trying to generate a version file within a docker container.