Increase number of runscope messages checked for e2e tests

Since builds run in parallel, the e2e tests sometimes fail
because their runscope messages are pushed down by other builds.

We currently test against 7 ruby versions, and a maximum of 2 builds are
triggered per PR (push + PR build). Checking 20 items should be
sufficient to handle this workload.

spec/helpers/runscope_client.rb

@@ -9,7 +9,7 @@ def initialize(api_token)
 
   def requests(bucket_key)
     with_retries(3) do
-      response = @conn.get("/buckets/#{bucket_key}/messages", count: 10)
+      response = @conn.get("/buckets/#{bucket_key}/messages", count: 20)
 
       raise "Runscope error. #{response.body}" unless response.status == 200