Bug fix for when no webdriver options are passed in.
Bug fix for when no webdriver options are passed in.

The following error is thrown when the "options" variable is undefined, making the first example in the README invalid.
```javascript
/home/user/Projects/test/node_modules/gulp-webdriver/node_modules/deepmerge/index.js:35
        Object.keys(src).forEach(function (key) {
               ^
TypeError: Object.keys called on non-object
    at Function.keys (native)
    at deepmerge (/home/user/Projects/test/node_modules/gulp-webdriver/node_modules/deepmerge/index.js:35:16)
    at DestroyableTransform._transform (/home/user/Projects/test/node_modules/gulp-webdriver/index.js:19:20)
    at DestroyableTransform.Transform._read (/home/user/Projects/test/node_modules/gulp-webdriver/node_modules/through2/node_modules/readable-stream/lib/_stream_transform.js:184:10)
    at DestroyableTransform.Transform._write (/home/user/Projects/test/node_modules/gulp-webdriver/node_modules/through2/node_modules/readable-stream/lib/_stream_transform.js:172:12)
    at doWrite (/home/user/Projects/test/node_modules/gulp-webdriver/node_modules/through2/node_modules/readable-stream/lib/_stream_writable.js:237:10)
    at writeOrBuffer (/home/user/Projects/test/node_modules/gulp-webdriver/node_modules/through2/node_modules/readable-stream/lib/_stream_writable.js:227:5)
    at DestroyableTransform.Writable.write (/home/user/Projects/test/node_modules/gulp-webdriver/node_modules/through2/node_modules/readable-stream/lib/_stream_writable.js:194:11)
    at write (/home/user/Projects/test/node_modules/gulp/node_modules/vinyl-fs/node_modules/through2/node_modules/readable-stream/lib/_stream_readable.js:623:24)
    at flow (/home/user/Projects/test/node_modules/gulp/node_modules/vinyl-fs/node_modules/through2/node_modules/readable-stream/lib/_stream_readable.js:632:7)
```
webdriverio/gulp-webdriver