Fixed issue where npmVersion is ignored

[kevinamasur]

Right now when I specify an npmVersion and set download = true the npmVersion I specified is being installed but not being used by NpmTasks. Instead, the npm bundled with the node setup is used.

I think the npmVersion is always getting resolved to null in the conditional.

node {
	npmVersion = '5.8.0'
	download = true
}

task printNpmConfigInfo(type: NpmTask) {
 args = ['config', 'list']
}

When running with the above the configured npm run is 6.5 located in the nodejs bin folder, not the defined npm.

user-agent = "npm/6.5.0 node/v6.9.1 darwin x64"

; node bin location = /.gradle/nodejs/node-v6.9.1-darwin-x64/bin/node