Use enumerable-statistics >= 2.0.1

enumerable-statistics 2.0.0 has a bug of hitogram which is the cause
of the issue #24.

Author: mrkn

unicode_plot.gemspec

@@ -29,7 +29,7 @@ Gem::Specification.new do |spec|
   spec.executables = spec.files.grep(%r{^exe/}) {|f| File.basename(f) }
   spec.require_paths = ["lib"]
 
-  spec.add_runtime_dependency "enumerable-statistics", ">= 2.0.0.pre"
+  spec.add_runtime_dependency "enumerable-statistics", ">= 2.0.1"
 
   spec.add_development_dependency "bundler", ">= 1.17"
   spec.add_development_dependency "rake"