Event Listener leak in unit tests

In `oae-preview-processor/tests/test-previews.js`, the `beforeEach` will run `PreviewProcessorAPI.enable` before each test, but `PreviewProcessorAPI.disable` will run only on the `after` which causes this console warning:

```
      ✓ verify default link processing works (10871ms)
      ✓ verify default link processing checks if a url is embeddable in an iframe (8867ms)
      ✓ verify default link processing checks if a url is available over HTTPS (11979ms)
(node) warning: possible EventEmitter memory leak detected. 11 listeners added. Use emitter.setMaxListeners() to increase limit.
Trace
    at EventEmitter.addListener (events.js:160:15)
    at EventEmitter.module.exports.enable (/Users/branden/Source/oaeproject/Hilary/node_modules/oae-preview-processor/lib/api.js:79:14)
    at /Users/branden/Source/oaeproject/Hilary/node_modules/oae-preview-processor/tests/test-previews.js:249:32
    at /Users/branden/Source/oaeproject/Hilary/node_modules/oae-preview-processor/lib/test/util.js:74:28
    at null.<anonymous> (/Users/branden/Source/oaeproject/Hilary/node_modules/oae-util/lib/mq.js:522:16)
    at EventEmitter.emit (events.js:117:20)
    at exports.Promise.emitSuccess (/Users/branden/Source/oaeproject/Hilary/node_modules/amqp/lib/promise.js:49:13)
    at Queue.Channel._handleTaskReply (/Users/branden/Source/oaeproject/Hilary/node_modules/amqp/lib/channel.js:69:20)
    at Queue._onMethod (/Users/branden/Source/oaeproject/Hilary/node_modules/amqp/lib/queue.js:355:29)
    at Queue.Channel._onChannelMethod (/Users/branden/Source/oaeproject/Hilary/node_modules/amqp/lib/channel.js:85:12)
      ✓ verify default link processing can handle HEAD failures (4063ms)
```