Explicitly tell gdb the python executable to use

Without this change I'm getting this on a system that has also python3 installed, when I try to attach() to a process:

    Traceback (most recent call last):
      File "<console>", line 1, in <module>
      File ".../pyringe/repl.py", line 157, in Attach
        self.inferior.Reinit(pid)
      File ".../pyringe/inferior.py", line 464, in Reinit
        self.__init__(pid, auto_symfile_loading)
      File ".../pyringe/inferior.py", line 436, in __init__
        self.StartGdb()
      File ".../pyringe/inferior.py", line 483, in StartGdb
        self._gdb.Attach(self.position)
      File ".../pyringe/inferior.py", line 196, in <lambda>
        return lambda *args, **kwargs: self._Execute(name, *args, **kwargs)
      File ".../pyringe/inferior.py", line 323, in _Execute
        result_string = self._Recv(timeout)
      File ".../pyringe/inferior.py", line 408, in _Recv
        raise ProxyError(exc_text)
    ProxyError: Traceback (most recent call last):
      File ".../pyringe/payload/gdb_service.py", line 34, in <module>
        import libpython
      File ".../pyringe/payload/libpython.py", line 58
        Py_TPFLAGS_HEAPTYPE = (1L << 9)
                                ^
    SyntaxError: invalid syntax

Curiosly enough `/usr/bin/env python` gives a python 2.7, but still gdb would try to run py3k.
This change does not make pyringe work for me: I still have a consistent TimeoutError. But IMO it should not hurt.