cassandra plugin - batch doesn't return promise

[cohesivejones]

I just tried to add dd-trace@v0.10.3 to my project and experienced the following error while calling a batch on the Cassandra client, returns error can't call then on undefined

The following test should expose the issue:

        it('should support batch queries with a promise', done => {
          const id = '1234'
          const queries = [
            { query: 'INSERT INTO test.test (id) VALUES (?)', params: [id] },
            `UPDATE test.test SET test='test' WHERE id='${id}';`
          ]

          agent
            .use(traces => {
              expect(traces[0][0]).to.have.property('resource', `${queries[0].query}; ${queries[1]}`)
            })
            .catch(done)

          client.batch(queries, { prepare: true }).then(done);
        })

Dependencies:

cassandra-driver: 3.5.0
dd-trace: 0.10.3