Hi,
this seems like a temporary error with the keyserver. Unfortunately GPG keyservers are notoriously slow and prone to errors.
Trying again a couple of times usually works, though I had no issues adding the key the first time:
Note that you can also use an alternative keyserver like hkp://pgp.mit.edu:80 if the Ubuntu one is giving you issues.
If all else fails, I exported the Bintray key manually, which you can download from here and add it with sudo apt-key add bintray.gpg.pub.
HTH,
Ivan