Fix an issue about per worker context count calculate

Author: hoppergee

lib/matrixeval/ruby/runner.rb

@@ -145,7 +145,8 @@ def report
       def parallel
         contexts = Context.all
 
-        contexts.each_slice(contexts.count / workers_count) do |sub_contexts|
+        per_worker_contexts_count = [(contexts.count / workers_count), 1].max
+        contexts.each_slice(per_worker_contexts_count) do |sub_contexts|
           threads << Thread.new do
             yield sub_contexts
           end