Fix postprocess convert command for alpha channels

On occasion the author would save his image as a .gif with transparency
when he (most likely) intended to leave the parts white. This was
already accounted for in the `convert` command, but the wrong flags were
being used. This should be fixed now (compare strip from 2002-09-11)

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

postprocess.sh

@@ -32,11 +32,11 @@ for date in "${dates[@]}"; do
 
     if [[ -z "$alt_text" ]]; then
         convert "$image_in" \
-            -background white -alpha background \
+            -background white -alpha remove -alpha off \
             "$image_out"
     else
         convert "$image_in" \
-            -background white -alpha background \
+            -background white -alpha remove -alpha off \
             -gravity center \
             -pointsize 12 -size 360x caption:"$alt_text" \
             -append \