sdl_ttf: use string dep for freetype

* also, style nit-picks

Formula/sdl_ttf.rb

@@ -1,15 +1,15 @@
-require 'formula'
+require "formula"
 
 class SdlTtf < Formula
-  homepage 'http://www.libsdl.org/projects/SDL_ttf/'
-  url 'http://www.libsdl.org/projects/SDL_ttf/release/SDL_ttf-2.0.11.tar.gz'
-  sha1 '0ccf7c70e26b7801d83f4847766e09f09db15cc6'
+  homepage "http://www.libsdl.org/projects/SDL_ttf/"
+  url "http://www.libsdl.org/projects/SDL_ttf/release/SDL_ttf-2.0.11.tar.gz"
+  sha1 "0ccf7c70e26b7801d83f4847766e09f09db15cc6"
 
   option :universal
 
-  depends_on 'pkg-config' => :build
-  depends_on 'sdl'
-  depends_on :freetype
+  depends_on "pkg-config" => :build
+  depends_on "sdl"
+  depends_on "freetype"
 
   def install
     ENV.universal_binary if build.universal?
@@ -18,6 +18,6 @@ class SdlTtf < Formula
                           "--disable-dependency-tracking",
                           "--prefix=#{prefix}",
                           "--disable-sdltest"
-    system "make install"
+    system "make", "install"
   end
 end