fixing spec - was failing

SSIA

```
[09:13] <vondruch> Failures:
[09:13] <vondruch>   1) BundlerExt#system_require non-strict mode should load the libraries using env var list
[09:13] <vondruch>      Failure/Error: defined?(Gem::Command).should be_true
[09:13] <vondruch>        expected: true value
[09:13] <vondruch>             got: nil
```