Adds parallel_tests rake task

Also moves parallel_tests out of development group so it's installed on travis

Comparison on an 8 core Mac:

```
$ bundle exec rake parallel_spec
746 examples, 0 failures, 10 pendings

Took 66 seconds (1:06)
```

Vs.

```
$ bundle exec rake spec
Finished in 3 minutes 4.8 seconds (files took 2.14 seconds to load)
746 examples, 0 failures, 10 pending
```

.rspec_parallel

@@ -0,0 +1,3 @@
+--format progress
+--format ParallelTests::RSpec::RuntimeLogger --out tmp/parallel_runtime_rspec.log
+--order random

Gemfile

@@ -11,10 +11,10 @@ gem 'puppet', ENV['PUPPET_VERSION'] || '~> 3.5'
 gem 'metadata-json-lint'
 gem 'retries', '~> 0.0.5'
 gem 'travis', '~> 1.8'
+gem 'parallel_tests'
 
 group :development do
   gem 'simplecov'
-  gem 'parallel_tests'
   gem 'ci_reporter'
   gem 'debugger', :platform => :mri_19
   gem 'debugger-pry', :platform => :mri_19

Rakefile

@@ -3,6 +3,8 @@ require 'rake'
 require 'puppetlabs_spec_helper/rake_tasks'
 require 'puppet-lint/tasks/puppet-lint'
 require 'puppet-syntax/tasks/puppet-syntax'
+require 'parallel_tests'
+require 'parallel_tests/cli'
 
 exclude_paths = [
   "pkg/**/*",
@@ -44,6 +46,12 @@ namespace :travis do
   end
 end
 
+desc "Parallel spec tests"
+task :parallel_spec do
+  Rake::Task[:spec_prep].invoke
+  ParallelTests::CLI.new.run('--type test -t rspec spec/classes spec/defines spec/unit spec/functions'.split)
+  Rake::Task[:spec_clean].invoke
+end
 
 task :default => [
   'travis:lint',