Downgrade to yarn 1.6.0.

Fixes #4793

This avoids the bug in yarn noted at https://github.com/yarnpkg/yarn/issues/5935, where a system path can override local paths. This bug can break executing commands from yarn in pipenv environments, for example.

package.json

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

yarn.lock

@@ -9190,9 +9190,9 @@ yargs@~3.10.0:
     decamelize "^1.0.0"
     window-size "0.1.0"
 
-yarn@^1.7.0:
-  version "1.7.0"
-  resolved "https://registry.npmjs.org/yarn/-/yarn-1.7.0.tgz#0076b9fde6010e01950526a609bc53bc175ef925"
+yarn@1.6.0:
+  version "1.6.0"
+  resolved "https://registry.npmjs.org/yarn/-/yarn-1.6.0.tgz#9cec6f7986dc237d39ec705ce74d95155fe55d4b"
 
 yeast@0.1.2:
   version "0.1.2"