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.

Author: jasonkarns

package.json

@@ -117,7 +117,8 @@
     "teenytest": "^5.1.1",
     "testdouble": "3.2.6",
     "testem": "^1.18.0",
-    "typescript": "^2.4.1"
+    "typescript": "^2.4.1",
+    "yarn": "^1.3.2"
   },
   "directories": {
     "doc": "./docs",