Update tox-travis requirement from ~=0.11 to ~=0.12 (#491)

Updates the requirements on [tox-travis](https://github.com/tox-dev/tox-travis) to permit the latest version.
- [Release notes](https://github.com/tox-dev/tox-travis/releases)
- [Changelog](https://github.com/tox-dev/tox-travis/blob/master/HISTORY.rst)
- [Commits](tox-dev/tox-travis@0.11...0.12)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Author: dependabot-preview[bot]

Pipfile

@@ -15,7 +15,7 @@ pytest = "~=4.0"
 pytest-cov = "~=2.8"
 
 tox = "~=3.6"
-tox-travis = "~=0.11"
+tox-travis = "~=0.12"
 
 httpretty = "~=0.9"
 python-dateutil = "~=2.8"