Upgrade yarn to 1.7 in devDependencies

package.json

@@ -54,7 +54,7 @@
     "lerna": "^2.4.0",
     "npm-run-all": "~4.1.1",
     "tslint": "~5.10.0",
-    "yarn": "^1.6.0"
+    "yarn": "^1.7.0"
   },
   "workspaces": [
     "dev_mode",

yarn.lock

@@ -8869,9 +8869,9 @@ yargs@~3.10.0:
     decamelize "^1.0.0"
     window-size "0.1.0"
 
-yarn@^1.6.0:
-  version "1.6.0"
-  resolved "https://registry.npmjs.org/yarn/-/yarn-1.6.0.tgz#9cec6f7986dc237d39ec705ce74d95155fe55d4b"
+yarn@^1.7.0:
+  version "1.7.0"
+  resolved "https://registry.npmjs.org/yarn/-/yarn-1.7.0.tgz#0076b9fde6010e01950526a609bc53bc175ef925"
 
 yeast@0.1.2:
   version "0.1.2"