Fixing column type to avoid java cast error

Current database throws:

```
ERROR com.ushahidi.swiftriver.core.api.controller.AbstractController - java.lang.ClassCastException: java.lang.Long cannot be cast to java.math.BigInteger
        at com.ushahidi.swiftriver.core.api.dao.impl.JpaIdentityDao.updateNewIdentityIndex(JpaIdentityDao.java:133)
```

This fixes things! :)