Fix bug where chips could temporarily swap colors
Fix bug where chips could temporarily swap colors

This fixes a strange bug where chips in the same column would sometimes
swap colors when the column was filled; the swap would correct the next
time the cursor was moved. This anomaly would only occur if the column
had been filled in a previous game.

This bug was ultimately caused by the presence of a Mithril-specific key
property on the virtual element for each chip. Removing this property on
the chip fixes the issue completely, however do note that the empty chip
slot key is still required for proper behavior.

caleb531/connect-four