Change yarn dep version range specification

In theory the caret `^` should work but as rochoa pointed out it does
not. So changing it (also for the sake of clarity).

INSTALL.md

@@ -5,7 +5,7 @@ Make sure that you have the requirements needed. These are
 
 - Core
   - Node.js >=6.9.x
-  - yarn >=0.27.5
+  - yarn >=0.27.5 <1.0.0
   - PostgreSQL >8.3.x, PostGIS >1.5.x
   - Redis >2.4.0 (http://www.redis.io)
   - Mapnik >3.x. See [Installing Mapnik](https://github.com/CartoDB/Windshaft#installing-mapnik).

package.json

@@ -62,6 +62,6 @@
   },
   "engines": {
     "node": ">=6.9",
-    "yarn": "^0.27.5"
+    "yarn": ">=0.27.5 <1.0.0"
   }
 }