add missing env arg

from backport

IPython/kernel/launcher.py

@@ -139,6 +139,7 b' def make_ipkernel_cmd(code, executable=None, extra_arguments=[], **kw):'
 def launch_kernel(cmd, stdin=None, stdout=None, stderr=None,
                         independent=False,
                         cwd=None, ipython_kernel=True,
+                        env=None,
                         **kw
                         ):
     """ Launches a localhost kernel, binding to the specified ports.