Re-enable import:model after each invocation in import:all

The elasticsearch:import:all task was only importing the first class that it attempted to import, rather than actually importing all.

Rake::Task#invoke only executes the task if it has not been invoked already, so by doing so in a loop, it only  actually gets invoked once. This pull request fixes this by calling Rake::Task#reenable after each invocation so that it can be invoked again in future iterations and import:all actually imports all.