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Minor fixes and improvements to the Vagrantfile
It may be better to use e.g. `distutils.spawn.find_executable` instead of hard-coding the paths to look up the executable (analogous to `which(1)`). See also:

- http://stackoverflow.com/questions/5226958/which-equivalent-function-in-python
- https://docs.python.org/2/distutils/apiref.html#module-distutils.spawn