Install Yarn as a dev dependency

Author: novemberborn

.travis.yml

@@ -5,4 +5,3 @@ node_js:
   - '4'
 before_install:
   - npm i -g npm
-  - npm i -g yarn

package.json

@@ -44,6 +44,7 @@
 		"ava": "*",
 		"dot-prop": "^4.1.0",
 		"temp-write": "^3.3.0",
-		"xo": "*"
+		"xo": "*",
+		"yarn": "^1.5.1"
 	}
 }