Allow nightly Yarn builds to be used (#6138)

* Allow nightly Yarn builds to be used

Fixes:
```
C:\src\babel (fix-it-fix-it-fix-it-fix-it) (babel)
λ yarn
yarn install v1.0.0-20170811.1240
[1/5] Validating package.json...
error babel@: The engine "yarn" is incompatible with this module. Expected version ">=0.27.5".
error Found incompatible module
info Visit https://yarnpkg.com/en/docs/cli/install for documentation about this command.
```

* Use Yarn 0.28.4 to fix build

circle.yml

@@ -10,7 +10,8 @@ machine:
 
 dependencies:
   pre:
-    - curl -o- -L https://yarnpkg.com/install.sh | bash
+    # Version number can be removed once 0.28.x or higher is released as stable
+    - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 0.28.4
   cache_directories:
     - ~/.yarn-cache
   override:

package.json

@@ -54,7 +54,7 @@
   "engines": {
     "node": ">= 4.x <= 8.x",
     "npm": ">= 2.x <= 5.x",
-    "yarn": ">=0.27.5"
+    "yarn": ">=0.27.5 || >=1.0.0-20170811"
   },
   "nyc": {
     "all": true,