Most probably, your private key is not in PKCS1 format. Try to create a copy of your key and convert it:
cp ~/.ssh/id_rsa ~/.ssh/id_rsa_pkcs1ssh-keygen -p -m PEM -f ~/.ssh/id_rsa_pkcs1
Then use id_rsa_pkcs1 key in Rocketeer.
id_rsa_pkcs1