Knex database spans not recorded when using async/await

**Describe the bug**
When a define a route wich uses knex to connect do the database, and use async await at the function definition, the apm agent does not send the sql span, not showing the sql info at the request timeline at kibana. The apm agent does not support async/await

**To Reproduce**
Steps to reproduce the behavior:
1. Use this config '...'
2. Then call '....'
3. Then do '....'
4. See error

**Expected behavior**
To show the sql span at the request timeline.

**Environment (please complete the following information)**
- OS: Linux
- Node.js version: v11.10.0
- APM Server version: 6.6.1
- Agent version: 2.5.1

**How are you starting the agent? (please tick one of the boxes)**
- [X] Calling `agent.start()` directly (e.g. `require('elastic-apm-node').start(...)`)
- [ ] Requiring `elastic-apm-node/start` from within the source code
- [ ] Starting node with `-r elastic-apm-node/start`

**Additional context**
Add any other context about the problem here.

- Agent config options

Click to expand

```
serviceName: 'service',
```

- `package.json` dependencies:

Click to expand

```
"elastic-apm-node": "^2.5.1",
"express": "^4.16.4",
"knex": "^0.16.3",
"mysql2": "^1.6.5"
```