Fix cover print truncation when coverage is 100%

[ferd]

When the option {cover_print_enabled, true} is set and that the code
coverage is 100%, the '%' will end up being truncated.

This patch makes it so that the truncation happens at 4 a characters
width so that instead of the following printouts:

some_mod  : 99%
some_other: 100

We instead show:

some_mod  :  99%
some_other: 100%

This solves issue #331 as reported by @lispking and @NineFX.

[ferd]

@Tuncer if you want to rubberstamp this one to avoid having me unilaterally moving my own patches in, that would be cool. It's minor stuff, literally a one-character change.

[ghost]

I'm not sure what "to rubberstamp" means, but this looks good to me.

[ferd]

It as in jest:

to give approval automatically or without consideration: to rubber-stamp the president's proposals.

Thanks for the review!