Lazily load fileutils and tmpdir

[deivid-rodriguez]

This change removes some fileutils redefintion warnings that happen when you boot a rails application that uses spring on a system with a default fileutils version, and a higher version of the fileutils gem available as well. This can happen, for example, on ruby 2.6.

Repro instructions are here: ruby/fileutils#22 (comment).

The short explanation of why this happens is that spring loads without using bundler, so the fileutils require uses rubygems version of Kernel.require which automatically activates the latest version of the corresponding gem if a require argument matches a default gem (like fileutils). Then rails itself also requires fileutils, but at that point bundler is already loaded, and bundler does not change Kernel.require at all, hence the default version of fileutils is chosen this time.

By lazily loading fileutils, we make sure that bundler is always loaded before fileutils, and thus the version that ends up being required is consistent.

The explanation of why also lazily loading tmpdir is because tmpdir also loads fileutils.

[deivid-rodriguez]

CI is terribly slow on TravisCI now. Should we migrate to a different CI?

[deivid-rodriguez]

Rebased now that #617 was merged!

[deivid-rodriguez]

Thanks a lot!

[pjg]

How about cutting a patch release with this change?