NO-JIRA use pip-installed tox instead of system-wide in Travis CI (#1473)

* NO-JIRA use pip-installed tox instead of system-wide in Travis CI

```
75: Test command: /usr/bin/tox
[...]
75: py38 create: /home/travis/build/apache/qpid-dispatch/build/tests/.tox/py38
75: ERROR: invocation failed (exit code 1), logfile: /home/travis/build/apache/qpid-dispatch/build/tests/.tox/py38/log/py38-0.log
```
1 file changed