#{prefix} should be interpolated

To extract a variable "prefix", use "%W" instead of "%w".
Using "%w", installation fails.

Closes Homebrew/homebrew#17429.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/mecab-ipadic.rb

@@ -8,7 +8,7 @@ class MecabIpadic < Formula
   depends_on "mecab"
 
   def install
-    args = %w[
+    args = %W[
       --disable-debug
       --disable-dependency-tracking
       --prefix=#{prefix}