Update the address for downloading libpng #3635

The FTP server what we get libpng from only keeps the latest version in its main folder. Older versions are in the "history" folder. Apparently version 1.6.9 has been released, so 1.6.8 has moved to the history folder.

Automatic sanity-testing: PASSED, see http://jenkins.bluematt.me/pull-tester/12edce56138fa8aa21c755f4dc2c16daebdab2a8 for binaries and test log. This test script verifies pulls every time they are updated. It, however, dies sometimes and fails to test properly. If you are waiting on a test, please check timestamps to verify that the test.log is moving at http://jenkins.bluematt.me/pull-tester/current/ Contact BlueMatt on freenode if something looks broken.

Seems there is nothing shocking in 1.6.9 that would prompt us to upgrade the dependency.

Changes since the last public release (1.6.8):
  Bookkeeping: Moved functions around (no changes). Moved transform
    function definitions before the place where they are called so that
    they can be masde static. Move the intrapixel functions and the
    grayscale palette builder out of the png?tran.c files. The latter
    isn't a transform function and is no longer used internally, and the
    former MNG specific functions are better placed in pngread/pngwrite.c
  Made transform implementation functions static. This makes the internal
    functions called by png_do_{read|write}_transformations static. On an
    x86-64 DLL build (Gentoo Linux) this reduces the size of the text
    segment of the DLL by 1208 bytes, about 0.6%. It also simplifies
    maintenance by removing the declarations from pngpriv.h and allowing
    easier changes to the internal interfaces.
  Rebuilt configure scripts with automake-1.14.1 and autoconf-2.69
    in the tar distributions.
  Added checks for libpng 1.5 to pngvalid.c.  This supports the use of
    this version of pngvalid in libpng 1.5
  Merged with pngvalid.c from libpng-1.7 changes to create a single
    pngvalid.c
  Removed #error macro from contrib/tools/pngfix.c (Thomas Klausner).
  Merged pngrio.c, pngtrans.c, pngwio.c, and pngerror.c with libpng-1.7.0
  Merged libpng-1.7.0 changes to make no-interlace configurations work
    with test programs.
  Revised pngvalid.c to support libpng 1.5, which does not support the
    PNG_MAXIMUM_INFLATE_WINDOW option, so #define it out when appropriate in
    pngvalid.c
  Allow unversioned links created on install to be disabled in configure.
    In configure builds 'make install' changes/adds links like png.h
    and libpng.a to point to the newly installed, versioned, files (e.g.
    libpng17/png.h and libpng17.a). Three new configure options and some
    rearrangement of Makefile.am allow creation of these links to be disabled.
  Removed potentially misleading warning from png_check_IHDR().
  Updated scripts/makefile.* to use CPPFLAGS (Cosmin).
  Added clang attribute support (Cosmin).
  Quiet an uninitialized memory warning from VC2013 in png_get_png().