Cairo/SVG test failure

[springmeyer]

In the process of moving to expect boost 1.59. Looks like this will require an update to this test:

  3) mapnik.VectorTile  should read back the vector tile and render a cairo svg with it:

      Uncaught AssertionError: svg diff 3356 not less that 50
      + expected - actual

      -false
      +true

      at test/vector-tile.test.js:1692:24