do not load rspec task if no rspec available
do not load rspec task if no rspec available

On production appliances, rspec gem is not installed
This was causing an error like:

```
rake aborted!
NameError: uninitialized constant RSpec
/var/www/miq/vmdb/lib/tasks/spec_evm.rake:10:in block (2 levels) in <top (required)>'
/var/www/miq/vmdb/lib/tasks/spec_evm.rake:2:inblock in '
/var/www/miq/vmdb/lib/tasks/spec_evm.rake:1:in <top (required)>'
/var/www/miq/vmdb/Rakefile:8:in'
/var/www/miq/vmdb/script/rake:15:in <top (required)>'
/var/www/miq/vmdb/script/rails:6:inrequire'
/var/www/miq/vmdb/script/rails:6:in `'
(See full trace by running task with --trace)
```

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