Replace depricated os.popen with subprocess in call_shell_wrapper

[joelferrier]

profilers/utils.py contains an os.popen call which which was deprecated in python2.6.
We should consider replacing the os.popen call with the subprocess module down the line to support profiling python 3 runtimes.

This is not a requirement for the MVP but should be tracked for once the MVP is complete.

[andrewkrug]

Feel free to submit a pull that makes it forward compatible. Still have to maintain 2.7 support in this case.