Add yarn as devDep
Add yarn as devDep

In order for the package to be installable via git (and to remove dist
from git), then the `prepare` script will need to be runnable from the
user's machine. Which means `yarn` needs to be available to npm users.

testdouble/testdouble.js,testdouble/testdouble.js,testdouble/testdouble.js