Allow the simulator to update a single app row when that's all that's necessary.
Added try/catch when checking for ps in PATH
Merge pull request #703 from bkase/file_io_ignore_exceptions

Added try/catch when checking for ps in PATH. Fixes #702
fix up receipt test for single app update
style nits
Merge pull request #706 from nickdesaulniers/update-single-app

Update single app