Fix failing test

Travis CI builds are failing with:

```
Failures:
  1) NETBuildpack::Util::RunCommand returns an error exit code
     Failure/Error: expect(exit_value).to eq(1)
       
       expected: 1
            got: 0
       
       (compared using ==)
     # ./spec/net_buildpack/util/run_command_spec.rb:33:in `block (2 levels) in <module:Util>'
```

This seems to be because `ls /this/doesnt/exist/foo/bar.baz` doesn't error with an exit code of 1 as expected.