Fix for custom install path can't link file of bin-dir

See issue https://github.com/composer/composer/issues/2230

If the library of custom install have some code like:
``` json
{
"bin" : [
    "bin/merge-assets"
  ]
}
```
while install fail.

Output error message like:
```
  [InvalidArgumentException]
  $from (~/Sites/test4/bin/merge-assets) and $to (libraries/test//bin/mer
  ge-assets) must be absolute paths.
```