travis: install hicolor-icon-theme for gtk3/test/

GitHub: #200

It may resolve the following test:

    Failure:
      <["emacs22",
       "display",
       "emacs23-document",
       "pstree16",
       "pstree32",
       "debian-logo",
       "chromium-browser",
       "python",
       "erlang",
       "oracle_java7",
       "openjdk-7",
       "openjdk-6",
       "javaws",
       "cmake",
       "emacs",
       "emacs-document",
       "python2.7",
       "emacs23"]> expected to be
      include?
      <"undo">.
    test_icons(TestGtkIconTheme)
    /home/travis/build/ruby-gnome2/ruby-gnome2/gtk3/test/test_gtk_icon_theme.rb:27:in `test_icons'
         24:   end
         25:
         26:   def test_icons
      => 27:     assert_operator(@theme.icons, :include?, "undo")
         28:     assert_operator(@theme.icons("Actions"), :include?, "remove")
         29:   end
         30: end