Fixed minimal sizing of the 3 lower boxes

[carloskelly13]

Minor fix to allow boxes to have a -1 offset to fit all three boxes in one row depending on the sizing of the terminal window.

Depending on the size of the window, the three sizes of 34%, 34% and 33% can cause an overflow. Testing different sizing configurations, 34%-1, 34%-1 and 33% had the most consistent results of always showing all three boxes on the same row and it consuming most of the full width.

Tested in both Terminal and iTerm.

Addresses #174

cc @ryan-roemer @kenwheeler

[ryan-roemer]

code looks good. Haven't tested locally -- let me know if you'd like me to...

[carloskelly13]

@ryan-roemer I would since it's visual and it'd be great to see if the fix holds up to other terminal configs.

The command is webpack-dashboard --minimal