update INSTALL document based on #650; according to #651

INSTALL

 * Prerequisites
   
   1. Make sure you have bison (http://www.gnu.org/software/bison/) installed in your system. 
-  2. Make sure you have ruby installed in your system. (Required by Minirake.)
+  2. Make sure you have ruby installed in your system (required to build).
 
 * Compilation and Installation
 
@@ -14,6 +14,10 @@
        * lib
        * include
 
+      You can directory invoke 'minirake' as the following.
+
+        $ ruby ./minirake
+
 If an error occurs when compiling mruby, it will be helpful for others if you
 send a detailed report to the developers that includes the error log, machine,
 and OS type.

README.md

@@ -57,7 +57,11 @@ See the INSTALL file.
 
 To run the tests, execute the following from the project's root directory.
 
-    $ ./minirake test
+    $ make test
+
+Or
+
+    $ ruby ./minirake test
 
 ## Customization