Add ulimit -u to all cellranger processes

Please fill in the appropriate checklist below (delete those which are not relevant). These are the most common things requested on pull requests.

PR checklist

• This comment contains a description of changes (with reason)
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated
• CHANGELOG.md is updated
• README.md is updated
• LICENSE.md is updated with new contributors

• Close issue
  Closes #44 (closed)

/cc @ghenry