Explicitly tell gdb the python executable to use

Without this change I'm getting this on a system that has also python3 installed:

  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.