Fix npmName prompt: true/false behaviour reversed
Fix npmName prompt: true/false behaviour reversed

Previously, if a name was available in npm, you would be contantly
reprompted to select a different name. And on the flipside, if a name
was NOT available, the generator would proceed as though it was

stevemao/generator-node,zckrs/generator-node,davewasmer/generator-node,rhettl/generator-node,RobertJGabriel/generator-node,yeoman/generator-node,thehobbit85/generator-node,iamstarkov/generator-node,canercandan/generator-node