When a PR title or commit summary contains quotation marks, they were
not escaped when injected into the JSON attachment. This caused
a message like

    unable to unmarshal webhook message: invalid character 't' after object key:value pair

when used inside of a template containing this test:

```json
{
  "text": "*Status*: FAILURE\n*Repo*: {{ .RepositoryName }} | *Branch*: {{ .BuildBranch }} | *Author*: {{ .BuildAuthor }}\n*Build*: {{ .BuildLink }}\n*Message*: {{ .BuildMessage }}"
}
```

This change safens this use case. Other typical fields _shouldn't_ have
quotation marks in them, but it might be prudent down the line to
cleanse _all_ fields as they are injected into the JSON or provide
a method usable inside of the template that escapes correctly.