Use excludes instead of exclude in ES _source query

As of ES 5.0, you must use the plural name.

src/worker.js

@@ -287,7 +287,7 @@ export default class Job extends events.EventEmitter {
     const nowTime = moment().toISOString();
     const query = {
       _source : {
-        exclude: [ 'output.content' ]
+        excludes: [ 'output.content' ]
       },
       query: {
         constant_score: {

test/src/worker.js

@@ -278,7 +278,7 @@ describe('Worker class', function () {
         const excludedFields = [ 'output.content' ];
         const { body } = getSearchParams(jobtype);
         expect(body).to.have.property('_source');
-        expect(body._source).to.eql({ exclude: excludedFields });
+        expect(body._source).to.eql({ excludes: excludedFields });
       });
 
       it('should search by job type', function () {