Fixed unit test for '#installed?' method

While using clear `rbenv` with ruby 1.9.3-p484, I had the next error in the `rake` output:
```bash
$ bundle exec rspec --color --format progress
...

Failures:

  1) Chef::Sugar::Shell#installed? returns true if the given binary exists
     Failure/Error: expect(described_class.installed?('mongo')).to be_true
       expected: true value
            got: false
     # ./spec/unit/chef/sugar/shell_spec.rb:33:in `block (3 levels) in <top (required)>'

Finished in 0.19738 seconds
467 examples, 1 failure

Failed examples:

rspec ./spec/unit/chef/sugar/shell_spec.rb:30 # Chef::Sugar::Shell#installed? returns true if the given binary exists
```

So, I've fixed it. We should stub the method of `described_class` here, not a `File` class, right?

P.s. I really don't know why the Travis still passed this test before. (Probably, it has `mongo` preinstalled?)