Don't prepend factory specs if SPEC is set

If you want to run **one** specific spec, you use something like:

``` console
$ SPEC=specs/path/to_spec.rb rake
```

Currently, doing this runs the specific spec *twice*, once during the `factory_specs` task and again during the `specs` task. This is because the `SPEC` environment variable is overriding `factory_specs`'s defined `pattern`.

This commit prevents Suspenders's `Rakefile` from adding `factory_specs` as a prereq task to `specs` when the `SPEC` environment variable is present.