Enables RSpec/ExampleWording and fixes all issues

Both avoiding 'should' and repiting 'it' on the tests description
improves reading them and also makes all descriptions consistent.

Read about cop at http://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/ExampleWording

spec/models/ahoy/data_source_spec.rb

@@ -15,18 +15,18 @@ describe Ahoy::DataSource do
       create :ahoy_event, name: 'bar', time: time_3
     end
 
-    it 'should work without data sources' do
+    it 'works without data sources' do
       ds = described_class.new
       expect(ds.build).to eq x: []
     end
 
-    it 'should work with single data sources' do
+    it 'works with single data sources' do
       ds = described_class.new
       ds.add 'foo', Ahoy::Event.where(name: 'foo').group_by_day(:time).count
       expect(ds.build).to eq :x => ["2015-01-01", "2015-01-02"], "foo" => [2, 1]
     end
 
-    it 'should combine data sources' do
+    it 'combines data sources' do
       ds = described_class.new
       ds.add 'foo', Ahoy::Event.where(name: 'foo').group_by_day(:time).count
       ds.add 'bar', Ahoy::Event.where(name: 'bar').group_by_day(:time).count