Fix NoMethodError when graph_columns is set to 1

Setting `:graph_columns:` to 1 in gdash.yaml caused a `NoMethodError` to
be thrown when viewing a dashboard. The problem is that the monkey
patched `Array#in_groups_of` method returns the unmodified array when
called as `array.in_group_of(1)`, while clients are expecting to get an
array of 1 element arrays.