builds:output uses STDERR instead of STDOUT

[nightpool]

When running heroku builds:output, the build output is printed on STDERR, rather than STDOUT. This makes it harder to write the output to a file or process it using any other standard unix tools. I was able to work around it using 2>&1, but that's hard to remember and may not be obvious to less experienced users. Plus, it means actual STDERR messages, like deprecation warnings from node, get mixed in with the build output.