make `-check4update` work again

[xorpaul]

$ wget -q https://raw.githubusercontent.com/xorpaul/g10k-environment/benchmark/Puppetfile ; \
g10k -check4update -puppetfile 
Resolving Forge modules (25/25)    1s [====================================================================] 100%
Resolving Git modules (4/4)    0s [====================================================================] 100%
unTar(): error while Create() file: .fixtures.yml Error: open external_modules/apt/.fixtures.yml: no such file or directory

[xorpaul]

Maybe give it another parameter to convert the output to JSON