Add missing requires

# Description:

I'm getting some crashes when running tests for my gem: https://circleci.com/gh/decidim/decidim/54469.
Adding the missing requires that the error points to seems to fix them. The require in `dependency.rb` comes from the error message, the second require comes from the error message that still appears after adding the first require.

I tried to isolate this, but I couldn't. So the minimal reproduction would be:

```
git clone https://github.com/decidim/decidim
cd decidim
git checkout rails-5.1.5-and-pg-1.0
bundle install
bundle exec rspec spec/generator_spec.rb[1:1:1:1]
```

# Tasks:

- [x] Describe the problem / feature
- [ ] Write tests
- [x] Write code to solve the problem
- [ ] Get code review from coworkers / friends

I will abide by the [code of conduct](https://github.com/rubygems/rubygems/blob/master/CODE_OF_CONDUCT.md).

Add missing requires

I'm getting some crashes when running tests for my gem: https://circleci.com/gh/decidim/decidim/54469.
Adding the missing requires that the error points to seems to fix them. The require in `dependency.rb` comes from the error message, the second require comes from the error message that still appears after adding the first require.

I tried to isolate this, but I couldn't. So the minimal reproduction would be:

```
git clone https://github.com/decidim/decidim
cd decidim
git checkout rails-5.1.5-and-pg-1.0
bundle install
bundle exec rspec spec/generator_spec.rb[1:1:1:1]
```

- [x] Describe the problem / feature
- [ ] Write tests
- [x] Write code to solve the problem
- [ ] Get code review from coworkers / friends

I will abide by the [code of conduct](https://github.com/rubygems/rubygems/blob/master/CODE_OF_CONDUCT.md).

in at least rubygems 2.7.6 when two chef gems are installed (as an
example, but i think the problem is generalized to other gems with
two installed versions, but it repros nicely with two chef gems),
instead of pinning a version a bundler bug makes us explode with

/opt/chefdk/embedded/lib/ruby/site_ruby/2.4.0/rubygems/dependency.rb:284:in `matching_specs': uninitialized constant Gem::BundlerVersionFinder (NameError)

which is ruby/rubygems#2196

this works around the bug by doing the require here, protected with
exception handling to not blow up on rubygems versions that don't
have this class.

once 2.7.7 / 2.8.0 is released this code should probably be
deleted (at the same time the exception means it shouldn't hurt
anything).

Signed-off-by: Lamont Granquist <lamont@scriptkiddie.org>