Fix new Rubocop issues

lib/dalli/pipelined_getter.rb

@@ -167,7 +167,7 @@ def groups_for_keys(*keys)
       groups = @ring.keys_grouped_by_server(keys)
       if (unfound_keys = groups.delete(nil))
         Dalli.logger.debug do
-          "unable to get keys for #{unfound_keys.length} keys "\
+          "unable to get keys for #{unfound_keys.length} keys " \
             'because no matching server was found'
         end
       end

lib/rack/session/dalli.rb

@@ -170,7 +170,7 @@ def retrieve_pool_options(options)
       def ensure_connection_pool_added!
         require 'connection_pool'
       rescue LoadError => e
-        warn "You don't have connection_pool installed in your application. "\
+        warn "You don't have connection_pool installed in your application. " \
              'Please add it to your Gemfile and run bundle install'
         raise e
       end

test/utils/certificate_generator.rb

@@ -24,7 +24,7 @@ def self.ssl_args
 
   def self.clean
     [ROOT_CA_CERT_PATH, ROOT_CA_PK_PATH, MEMCACHED_CERT_PATH, MEMCACHED_PK_PATH].each do |path|
-      File.delete(path) if File.exist?(path)
+      FileUtils.rm_rf(path)
     end
   end