storyboard: use RGB instead of RGBA

Since Pillow 4.2.0, attempting to save an RGBA image as JPEG results in
an IOError.[1] We don't really need an alpha channel anyway (or do we?).

[1] https://github.com/python-pillow/Pillow/blob/master/docs/releasenotes/4.2.0.rst#removed-deprecated-items

src/storyboard/storyboard.py

@@ -411,7 +411,7 @@ def tile_images(images, tile, params=None):
         resize = False
 
     # start assembling images
-    canvas = Image.new('RGBA', (canvas_width, canvas_height), canvas_color)
+    canvas = Image.new('RGB', (canvas_width, canvas_height), canvas_color)
     y = ver_margin
     for row in range(0, rows):
         x = hor_margin