Address gradle deprecation warning

Address the following warning by putting the lpreview variable in gradle.properties;

```
Creating properties on demand (a.k.a. dynamic properties) has been deprecated and is scheduled to be removed in Gradle 2.0. Please read http://gradle.org/docs/current/dsl/org.gradle.api.plugins.ExtraPropertiesExtension.html for information on the replacement for dynamic properties.
```

[Unfortunately gradle doesn't auto-cast the value to a boolean so an explicit cast is needed.]