gistfile1.txt

curl -u 'jameswhite:I<3t3hK@wk!'\
     -X POST -d '{ "title": "${LAST_COMMIT_MSG}","body": "${LAST_COMMIT_MSG}","head": "${GITHUB_USERNAME}:lab","base": "lab"}' \
     -H "Content-Type: application/json" https://api.github.com/repos/eftsource/cfengine/pulls

Works as expected

--

curl -X POST -d '{ "title": "${LAST_COMMIT_MSG}","body": "${LAST_COMMIT_MSG}","head": "${GITHUB_USERNAME}:lab","base": "lab"}' \
     -H "Authorization: token ${PERSO_ACCESS_TOKEN}" \
     -H "Content-Type: application/json" https://api.github.com/repos/eftsource/cfengine/pulls

"Content-Type: application/json" https://api.github.com/repos/eftsource/cfengine/pulls
{
  "message": "Bad credentials",
  "documentation_url": "http://developer.github.com/v3"
}