Fix "everything passes, travis fails"

[mramato]

1. buildSandcastle is returning a stream that is passed to Promise.join. That means the promise resolves before the stream is finished. Instead it should be returning a promise created from the stream.

2. npm pack may be screwed with travis' console leading to random failures, redirect it's output to /dev/null to see if it addresses our random build failures

3. requirejs was occasionally being killed by travis for using too much memory, reduce concurrency during the build process to prevent this from happening.