use expect instead of should in specs

spec/models/supplier_spec.rb

@@ -5,12 +5,12 @@ describe Supplier do
 
   it 'has a unique name' do
     supplier2 = FactoryGirl.build :supplier, name: supplier.name
-    supplier2.should_not be_valid
+    expect(supplier2).to be_invalid
   end
 
   it 'has valid articles' do
     supplier = FactoryGirl.create :supplier, article_count: true
-    supplier.articles.all.each {|a| a.should be_valid }
+    supplier.articles.all.each {|a| expect(a).to be_valid }
   end
 
 end