Fix panic when the prefix is > 61 chars

[blalor]

The bar function subtracts 2 from the barSize; if barSize is < 2, drawing the progress bar will panic because you can't string.Repeat a negative length.

Running: [/src/config.sh]
meta tag not found on XX.XXXXXXXXX.XXX/XXX-XXX/XXXX/XXXXXXX-XXXX-XXXXXXX: expected a 200 OK got 403
panic: runtime error: makeslice: len out of range

goroutine 1 [running]:
strings.Repeat(0x857ee0, 0x1, 0xfffffffffffffffe, 0x0, 0x0)
    /usr/lib/golang/src/strings/strings.go:458 +0x68
github.com/appc/acbuild/Godeps/_workspace/src/github.com/coreos/ioprogress.func·004(0x0, 0x37ae506, 0x0, 0x0)
    /root/acbuild/gopath/src/github.com/appc/acbuild/Godeps/_workspace/src/github.com/coreos/ioprogress/draw.go:112 +0x270
github.com/appc/acbuild/registry.func·002(0x0, 0x37ae506, 0x0, 0x0)
    /root/acbuild/gopath/src/github.com/appc/acbuild/registry/fetch.go:431 +0x1e7

[cgonyeo]

Nice catch. I'll want to ultimately fix this in ioprogress, but might as well take care of it here until that happens.