Use the last line of the npm pack command

To fetch the name of the tarball.

Author: jtpio

jupyter_releaser/npm.py

@@ -24,7 +24,7 @@ def build_dist(package, dist_dir):
 
     if osp.isdir(package):
         basedir = package
-        tarball = osp.join(package, util.run("npm pack", cwd=package))
+        tarball = osp.join(package, util.run("npm pack", cwd=package).split('\n')[-1])
     else:
         basedir = osp.dirname(package)
         tarball = package