rbinstall.rb: set umask 077

* tool/rbinstall.rb: default umask to prohibit group/others from
  reading/writing/executing.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49841 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/rbinstall.rb

@@ -26,7 +26,7 @@ rescue LoadError
 end
 
 STDOUT.sync = true
-File.umask(0222)
+File.umask(077)
 
 def parse_args(argv = ARGV)
   $mantype = 'doc'