Use new invocations blacken task

.travis.yml

@@ -38,8 +38,6 @@ install:
   # TODO: follow invoke and split it up a bit so we're not pulling down
   # conflicting or unused-by-travis deps?
   - pip install -r dev-requirements.txt
-  # Black, only on 3.6, it literally cannot install on anything else =/
-  - "if [[ $TRAVIS_PYTHON_VERSION == '3.6' ]]; then pip install black==18.4a4; fi"
   # Sanity test of the Invoke layer, if that's busted everything is
   - inv --list
   # Sanity test of Fabric itself
@@ -51,9 +49,8 @@ before_script:
   - inv travis.make-sshable
 script:
   # Fast syntax check failures for more rapid feedback to submitters
-  # (Python 3.6 only; it doesn't run on anything less and there's no
-  # point running it multiple times either.)
-  - "if [[ $TRAVIS_PYTHON_VERSION == '3.6' ]]; then inv blacken --check; fi"
+  # (Travis-oriented metatask that version checks Python, installs, runs.)
+  - inv travis.blacken
   # I have this in my git pre-push hook, but contributors probably don't
   - flake8
   # Execute full test suite + coverage, as the new sudo-capable user