In Rails 5, `rake geocode:all` crashes:

```
$ rake geocode:all CLASS=Whatzit
rake aborted!
ArgumentError: comparison of Fixnum with nil failed
/Users/paul/.rvm/gems/ruby-2.2.2/gems/activerecord-5.0.0/lib/active_record/relation/batches.rb:217:in `<'
/Users/paul/.rvm/gems/ruby-2.2.2/gems/activerecord-5.0.0/lib/active_record/relation/batches.rb:217:in `block in in_batches'
/Users/paul/.rvm/gems/ruby-2.2.2/gems/activerecord-5.0.0/lib/active_record/relation/batches.rb:198:in `loop'
/Users/paul/.rvm/gems/ruby-2.2.2/gems/activerecord-5.0.0/lib/active_record/relation/batches.rb:198:in `in_batches'
/Users/paul/.rvm/gems/ruby-2.2.2/gems/activerecord-5.0.0/lib/active_record/relation/batches.rb:120:in `find_in_batches'
/Users/paul/.rvm/gems/ruby-2.2.2/gems/activerecord-5.0.0/lib/active_record/relation/batches.rb:58:in `find_each'
/Users/paul/.rvm/gems/ruby-2.2.2/gems/geocoder-1.2.2/lib/tasks/geocoder.rake:11:in `block (2 levels) in <top (required)>'
/Users/paul/.rvm/gems/ruby-2.2.2/gems/rake-11.2.2/exe/rake:27:in `<top (required)>'
/Users/paul/.rvm/gems/ruby-2.2.2/bin/ruby_executable_hooks:15:in `eval'
/Users/paul/.rvm/gems/ruby-2.2.2/bin/ruby_executable_hooks:15:in `<main>'
Tasks: TOP => geocode:all
(See full trace by running task with --trace)
```

The problem seems to be that `find_each` used to default the `batch:` param when it was explicitly set to nil, but now passes an explicit nil on through.

This patch fixes the issue by defaulting the batch size in the rake task.

Also split single rake task into several.

Use 'geocode' instead of 'fetch_coordinates!'.