use `unless` instead of `if not`.

mrbgems/mruby-enum-lazy/mrblib/lazy.rb

@@ -69,7 +69,7 @@ class Enumerator
 
     def reject(&block)
       Lazy.new(self){|yielder, val|
-        if not block.call(val)
+        unless block.call(val)
           yielder << val
         end
       }

mrblib/string.rb

@@ -133,7 +133,7 @@ class String
     if args.length == 2 && block
       block = nil
     end
-    if !block
+    unless block
       replace = replace.to_str
     end
     result = []

test/assert.rb

@@ -157,7 +157,7 @@ def assert_raise(*exp)
       msg = "#{msg}#{exp.inspect} exception expected, not"
       diff = "      Class: <#{e.class}>\n" +
              "    Message: #{e.message}"
-      if not exp.any?{|ex| ex.instance_of?(Module) ? e.kind_of?(ex) : ex == e.class }
+      unless exp.any?{|ex| ex.instance_of?(Module) ? e.kind_of?(ex) : ex == e.class }
         $mrbtest_assert.push([$mrbtest_assert_idx, msg, diff])
         ret = false
       end