Revert "Use python -m to call virtualenv in init_venv."

This reverts commit 40781b163e.

Some platforms (e.g. Ubuntu Trusty) don't have a python3-virtualenv, so we
should instead adjust the documentation to use the python2 one.

scripts/init_venv.py

@@ -149,7 +149,7 @@ def create_venv():
         sys_site = ['--system-site-packages']
     else:
         sys_site = []
-    subprocess.check_call([sys.executable, '-m', 'virtualenv'] + sys_site +
+    subprocess.check_call(['virtualenv'] + sys_site +
                           ['-p', sys.executable, g_path])