Log Message:
Update ruby-cairo to 1.15.5.

Release 1.15.5 (2017-01-14) Kouhei Sutou <kou@cozmixng.org>
===========================================================

Improvements
------------

  * Windows: Applied the patch at
    https://bugs.freedesktop.org/show_bug.cgi?id=99054 that fixes
    unexpected assertion failure in win32 surface.

Release 1.15.4 (2017-01-07) Kouhei Sutou <kou@cozmixng.org>
===========================================================

Improvements
------------

  * Supported cairo 1.15.4:
    * Added Cairo::PNGError.
    * Added Cairo::FreeTypeError.
    * Added Cairo::Win32GDIError.
    * Added Cairo::TagError.
    * Added Cairo::Tag::DEST.
    * Added Cairo::Tag::LINK.
    * Added Cairo::Context#tag.
    * Added Cairo::Context#begin_tag.
    * Added Cairo::Context#end_tag.
    * Added Cairo::PDFOutlineFlags.
    * Added Cairo::PDFOutline.
    * Added Cairo::PDFSurface#add_outline.
    * Added Cairo::PDFMetadata.
    * Added Cairo::PDFSurface#set_metadata.
    * Added Cairo::PDFSurface#page_label=.
    * Added Cairo::PDFSurface#set_thumbnail_size.