fix initializing without imageVersions set

[Demo](http://thejackalofjavascript.com/uploading-files-made-fun/) says, that minimal options are these:
```
var options = {
    tmpDir:  __dirname + '/../public/uploaded/tmp',
    uploadDir: __dirname + '/../public/uploaded/files',
    uploadUrl:  '/uploaded/files/',
    storage : {
        type : 'local'
    }
};
```

But when you initialize the module without ```imageVersions``` an error occurs:
```
.../blueimp-file-upload-expressjs/index.js:50
    Object.keys(opts.imageVersions).forEach(function(version) {
           ^
TypeError: Object.keys called on non-object
```

This simple PR fixes this issue with checking for ```opts.imageVersions``` before working with it.