Enable warnings when running RSpec from rake

- travis: Use rake to run specs to generate warnings.

.travis.yml

@@ -12,7 +12,7 @@ rvm:
 
 script:
   - bundle exec rubocop
-  - bundle exec rspec
+  - bundle exec rake spec
   - bundle exec rake install
   - bundle binstubs colorls
   - PATH=$PWD/bin:$PATH

Rakefile

@@ -3,7 +3,9 @@ require 'rubygems/tasks'
 Gem::Tasks.new
 
 require 'rspec/core/rake_task'
-RSpec::Core::RakeTask.new
+RSpec::Core::RakeTask.new(:spec) do |t|
+  t.rspec_opts = "--warnings"
+end
 
 require 'rubocop/rake_task'
 RuboCop::RakeTask.new do |task|