Bump cacheman TTL from 60 seconds to 1 hour (#210)

## Why are we doing this?

While investigating losing selfies #193 , it doth appear that [Cacheman's default TTL is 60 seconds](https://github.com/cayasso/cacheman/blob/6149f436268cedbec54633c81481ae7035411873/lib/index.js#L99-L104) but [it's not documented](cayasso/cacheman#37). So it was dropping data. This raises it to match our definition of a user session, one hour.

## Did you document your work?

This explicitly sets the TTL, which increases documentations

## How can someone test these changes?

Steps to manually verify the change:

1. Using the `master` branch
1. I started a bot
2. talked to the bot, noting the `count` log
```
lenses:messenger message.text user:1250872178269050 text: "fart" count: 1 seq: 8760
```
3. wait over a minute and talk again, the `count` will go back to `1`.
5. do the same thing with this branch, but the `count` will keep going up.

## What possible risks or adverse effects are there?

I haven't looked into cacheman's internals, but this is effectively the same as a memory leak. We can transition to another backend if this is a problem. As it is, we're not using a lot of RAM.

## What are the follow-up tasks?

* watch #193 

## Are there any known issues?

none

## Did the test coverage decrease?

same, initial values to cacheman are not part of the test suite.

app.js

@@ -11,10 +11,10 @@ const reqPromise = require('request-promise');
 const urlJoin = require('url-join');
 const conversationLogger = require('./conversationLogger');
 
-const cache = new Cacheman('sessions');
-
 const SESSION_TIMEOUT_MS = 3600 * 1000;  // 1 hour
 
+const cache = new Cacheman('sessions', {ttl: SESSION_TIMEOUT_MS / 1000});
+
 const internals = {};