SVG viewbox shouldn't be required, default to 2048x2048.

[13rac1]

Color SVG files shouldn't require a viewbox attribute. Assume 2048x2048 viewbox attribute if missing.

Note: Viewbox is required in B&W SVGs to calculate glyph width for proportional fonts.

Found while working on: 13rac1/emojione-color-font#50

Traceback (most recent call last):
  File "SCFBuild/bin/scfbuild", line 17, in <module>
    sys.exit(main())
  File "/home/example/development/emojione-color-font/SCFBuild/scfbuild/main.py", line 124, in main
    return builder.run()
  File "/home/example/development/emojione-color-font/SCFBuild/scfbuild/builder.py", line 74, in run
    self.add_color_svg()
  File "/home/example/development/emojione-color-font/SCFBuild/scfbuild/builder.py", line 102, in add_color_svg
    svg_transform = self.create_color_transform(filepath)
  File "/home/example/development/emojione-color-font/SCFBuild/scfbuild/builder.py", line 186, in create_color_transform
    svg_height, svg_width = util.get_dimensions(filepath)
  File "/home/example/development/emojione-color-font/SCFBuild/scfbuild/util.py", line 60, in get_dimensions
    dims = root.attrib['viewBox'].split(' ')
KeyError: u'viewBox'