Use Python-native methods to check if ipfs is installed instead of subprocess

[machawk1]

Per #84, we use ipfs --version via Python subprocess to check if ipfs is installed before firing off subsequent ipwb procedures. @ibnesayeed mentioned that Py2's find_executable and Py3's shutil.which would incur much less expense to the system and it would be better to keep the call in Python instead of forking processes.