Enable RSpec/PredicateMatcher cop & fix all issues

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

.rubocop.yml

@@ -153,3 +153,6 @@ RSpec/NotToNot:
 
 RSpec/OverwritingSetup:
   Enabled: true
+
+RSpec/PredicateMatcher:
+  Enabled: true

spec/models/geozone_spec.rb

@@ -16,27 +16,27 @@ describe Geozone do
     let(:geozone) { create(:geozone) }
 
     it "is true when not linked to other models" do
-      expect(geozone.safe_to_destroy?).to be_truthy
+      expect(geozone).to be_safe_to_destroy
     end
 
     it "is false when already linked to user" do
       create(:user, geozone: geozone)
-      expect(geozone.safe_to_destroy?).to be_falsey
+      expect(geozone).not_to be_safe_to_destroy
     end
 
     it "is false when already linked to proposal" do
       create(:proposal, geozone: geozone)
-      expect(geozone.safe_to_destroy?).to be_falsey
+      expect(geozone).not_to be_safe_to_destroy
     end
 
     it "is false when already linked to spending proposal" do
       create(:spending_proposal, geozone: geozone)
-      expect(geozone.safe_to_destroy?).to be_falsey
+      expect(geozone).not_to be_safe_to_destroy
     end
 
     it "is false when already linked to debate" do
       create(:debate, geozone: geozone)
-      expect(geozone.safe_to_destroy?).to be_falsey
+      expect(geozone).not_to be_safe_to_destroy
     end
   end
 end