moves libpng from optional to required

Author: katyhuff

INSTALL

@@ -196,6 +196,12 @@ six 1.3 or later
     Python 2/3 compatibility library.  This is also a dependency of
     :term:`dateutil`.
 
+libpng 1.2 (or later)
+    library for loading and saving :term:`PNG` files (`download
+    <http://www.libpng.org/pub/png/libpng.html>`__). libpng requires
+    zlib. If you are a Windows user, you can ignore this because we
+    build support into the matplotlib single-click installer
+
 
 **Optional GUI frameworks**
 
@@ -239,13 +245,6 @@ ImageMagick
     user, you can ignore this since we build support into the
     matplotlib single click installer.
 
-
-libpng 1.2 (or later)
-    library for loading and saving :term:`PNG` files (`download
-    <http://www.libpng.org/pub/png/libpng.html>`__). libpng requires
-    zlib. If you are a Windows user, you can ignore this because we
-    build support into the matplotlib single-click installer
-
 **Required libraries that ship with matplotlib**
 
 :term:`agg` 2.4