Make config file optional

This change allows you to configure Protractor entirely from within the Gulp task, like:

```
var protractor = require('gulp-protractor').protractor;

gulp.task('protractor', function(done) {
  gulp.src('e2e-tests/**/*.js')
    .pipe(protractor({
      args: [
        '--allScriptsTimeout', '11000',
        '--capabilities.browserName', 'chrome',
        '--baseUrl', 'http://localhost:8000',
        '--framework', 'jasmine'
      ]
    }))
    .on('end', done);
});
```