Use the versioned python command as PEP-394 suggests

I'm on Arch and tried building opentsdb and ran into the same problem as in #83. I updated the command to python2 and the build works just fine. I'm aware that it's Arch that doesn't follow convention to have python 2 as the default but the command python2 should be more portable as it's suggested to use this in [this](http://legacy.python.org/dev/peps/pep-0394/) PEP.

> so python should be used in the shebang line only for scripts that are source compatible with both Python 2 and 3.