Fix app boot for Ruby 2.4

I have a test app that was on Ruby 2.4. When I pulled Rails master the
app no longer would boot because of this change and I saw the following
error:

```
SyntaxError:
/Users/eileencodes/open_source/real_rails/railties/lib/rails/all.rb:18:
syntax error, unexpected keyword_rescue, expecting keyword_end
rescue LoadError
      ^
```

Ruby 2.4 doesn't support removing redundant begins so the real issue is
that this app is on Ruby 2.4 and not on Ruby 2.5. But it's super
confusing for a user to understand the reason the app is failing to boot
is because we need Ruby 2.5.

I added this redundant begin back because we need to give a clearer
error message.

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