Fix/userid escaping

A customer reported that any `userId` with a `/` in it would not show up in Customer.io. We found that this is because the tracking url contains the `userId`:
https://github.com/segmentio/integration-customerio/blob/cc203c3e5046f261e3897bfdf06e5f8fd17913cf/lib/index.js#L78

However, escaping at `/` results in a `%2F`. A `%2F` blocks the data from appearing in Segment, and a `%` by itself breaks Customer.io's interface: https://cloudup.com/cnL1sfhlI1U

At this point, it's unclear what the proper way to escape a `userId` for Customer.io is. So we should wait on merging this pull request until Customer.io provides guidance.