Change to `appraisal install`

executing to `rake appraisal:install` log

```
`rake appraisal:install` task is deprecated and will be removed soon. Please use `appraisal install`.
```

CONTRIBUTING.md

@@ -43,7 +43,7 @@ will be asked to rewrite them before we'll accept.
 ### Bootstrapping your test suite:
 ### Bootstrapping your test suite:
 
 
     bundle install
     bundle install
-    bundle exec rake appraisal:install
+    bundle exec appraisal install
 
 
 This will install all the required gems that requires to test against each
 This will install all the required gems that requires to test against each
 version of Rails, which defined in `gemfiles/*.gemfile`.
 version of Rails, which defined in `gemfiles/*.gemfile`.