fix runtox.py failure when 'tox' is not available on the current system path

Now just assumes that the tox module is available in the executing Python
environment.

--HG--
branch : jurko/tox_usage

runtox.py

@@ -4,7 +4,7 @@ import subprocess
 import sys
 
 if __name__ == "__main__":
-    subprocess.call(["tox",
+    subprocess.call([sys.executable, "-m", "tox",
                      "-i", "ALL=https://devpi.net/hpk/dev/",
                      "--develop",] + sys.argv[1:])