Use beta version in README

@melissawahnish noticed that while we are on beta v5 will not be found.

[ci skip]

README.md

@@ -159,7 +159,7 @@ Paperclip is distributed as a gem, which is how it should be used in your app.
 Include the gem in your Gemfile:
 
 ```ruby
-gem "paperclip", "~> 5.0"
+gem "paperclip", "~> 5.0.0.beta1"
 ```
 
 Or, if you want to get the latest, you can get master from the main paperclip repository: