spec_clean does not fail if it has already been run

I should be able to run `rake spec_clean` as many times as I'd like.

**before**:

```
➜  puppet-pulp  rake spec_prep
➜  puppet-pulp  rake spec_clean
➜  puppet-pulp  rake spec_clean
rake aborted!
No such file or directory - spec/fixtures/modules/pulp
/Users/hnewton/Projects/puppetlabs_spec_helper/lib/puppetlabs_spec_helper/rake_tasks.rb:90:in `block (2 levels) in <top (required)>'
/Users/hnewton/Projects/puppetlabs_spec_helper/lib/puppetlabs_spec_helper/rake_tasks.rb:89:in `each'
/Users/hnewton/Projects/puppetlabs_spec_helper/lib/puppetlabs_spec_helper/rake_tasks.rb:89:in `block in <top (required)>'
/Users/hnewton/.rvm/gems/ruby-1.9.3-p448@puppet-pulp/bin/ruby_noexec_wrapper:14:in `eval'
/Users/hnewton/.rvm/gems/ruby-1.9.3-p448@puppet-pulp/bin/ruby_noexec_wrapper:14:in `<main>'
Tasks: TOP => spec_clean
(See full trace by running task with --trace)
```

**after**:
```
➜  puppet-pulp  rake spec_prep
➜  puppet-pulp  rake spec_clean
➜  puppet-pulp  rake spec_clean
```