Add `--pre` to the recommended command to install jekyll-import.

Since jekyll-import has not released any versions that are non-beta,
running `gem install jekyll-import` as currently recommended by the
error message results in a new error message:

    ERROR:  Could not find a valid gem 'jekyll-import' (>= 0) in any
repository
    ERROR:  Possible alternatives: jekyll-import

This is confusing and unhelpful, so until jekyll-import releases a
non-beta version, we should recommend using `--pre`.

I know the contribution guides say all PRs need tests, but I could not find any currently existing tests that assert on the output of the command, and I found myself really wanting to just use [aruba](https://github.com/cucumber/aruba) but I'm loathe to add dependencies to projects without asking.

So if you'd be fine with me using aruba to write tests for this, I would be happy to. If you have another idea rather than using aruba, please give me some guidance. If this is not worth testing, well, here you are :)