Do not die if last require failed

There is currently a bug in Class::Refresh.

Suppose a module has been required, but that require failed:
```
foreach my $module (qw( ModA ModB ModC )) {
    if (eval "require $module; 1") {  # now using this module
        last;
    }
}
``` 

This is acceptable enough perl.  ```$INC{'ModA.pm'}``` may be ```undef``` to indicate that the search failed.

Class::Refresh should not die when it finds a module in ```%INC``` which is ```undef``` and it fails to load, because presumably that error was already handled by whatever code tried to load it.  Otherwise, there would have been an exception and we wouldn't be trying to ```Class::Refresh->refresh``` at all!

Let me know if you have any questions or want any clarification!