send rspec options to parallel_tests using -o

Running guard-rspec with notifications and parallel_tests, I received the following error:
```
15:59:23 - INFO - Guard uses Growl to send notifications.
15:59:28 - INFO - Guard::RSpec is running
15:59:28 - INFO - Running all specs
.../gems/parallel_tests-0.10.0/lib/parallel_tests/cli.rb:65:in `parse_options!': invalid option: -r (OptionParser::InvalidOption)
	from .../gems/parallel_tests-0.10.0/lib/parallel_tests/cli.rb:6:in `run'
	from .../gems/parallel_tests-0.10.0/bin/parallel_rspec:5:in `<top (required)>'
	from .../bin/parallel_rspec:19:in `load'
	from .../bin/parallel_rspec:19:in `<main>'
	from .../bin/ruby_noexec_wrapper:14:in `eval'
	from .../bin/ruby_noexec_wrapper:14:in `<main>'
```
Tracked it down to the options passed to parallel_tests. Need to wrap the rspec options with -o