fix 'rm: gemfiles/*.lock: No such file or directory' error when running rake

Rakefile

@@ -11,7 +11,7 @@ task :all do |t|
   if ENV['BUNDLE_GEMFILE']
     exec('rake spec cucumber')
   else
-    exec("rm gemfiles/*.lock")
+    exec("rm -f gemfiles/*.lock")
     Rake::Task["appraisal:gemfiles"].execute
     Rake::Task["appraisal:install"].execute
     exec('rake appraisal')