Allow bootstrap to be invoked via python2

This means that on most Linux distributions where python3 is the default, we
can invoke `python2 script/bootstrap.py` and have it all work

common.gypi

@@ -15,7 +15,7 @@
     'openssl_fips': '',
     'openssl_no_asm': 1,
     'node_release_urlbase': 'https://atom.io/download/atom-shell',
-    'node_byteorder': '<!(python -c "import sys; print sys.byteorder")',
+    'node_byteorder': '<!(node <(DEPTH)/tools/get-endianness.js)',
     'node_target_type': 'shared_library',
     'node_install_npm': 'false',
     'node_prefix': '',