SpiderMonkey upgrade details

Latest revision: 5ecd532a167e

Under mozjs:

cp -R ~/mozilla-central/js .
cp -R ~/mozilla-central/mfbt .
cp -R ~/mozilla-central/build .

Comment out builtin(include, ../../build/autoconf/python-virtualenv.m4)dnl in js/src/aclocal.m4.

Second attempt:
```
cd ~/mozilla-central
make -C objdir/js/src source-package
cd ~/servo/src/support/mozjs
rm -rf .
cp ~/mozilla-central/objdir/mozjs-31.0.1.tar.bz2 .
tar xvjf mozjs-31.0.1.tar.bz2 --strip 1
```