Don't throw exception when gyp fails.

This makes output cleaner when we got a gyp error.

script/update.py

@@ -31,7 +31,7 @@ def update_gyp():
   python = sys.executable
   if sys.platform == 'cygwin':
     python = os.path.join('vendor', 'python_26', 'python.exe')
-  subprocess.check_call([python, gyp,
+  subprocess.call([python, gyp,
                    '-f', 'ninja', '--depth', '.', 'atom.gyp',
                    '-Icommon.gypi', '-Ivendor/brightray/brightray.gypi',
                    '-Dtarget_arch=ia32', '-Dlibrary=static_library'])