Switch from bash to sh for black wrapper.

scripts/black

@@ -1,13 +1,7 @@
-#!/usr/bin/env bash
+#!/usr/bin/env sh
 
-set -e
-
-python_version=$(python --version)
-
-if [[ "$python_version" =~ "Python 3.5." ]]; then
-    echo "Skipping black due to lack of support for $python_version."
-elif [[ "$python_version" =~ "Python 3.4." ]]; then
-    echo "Skipping black due to lack of support for $python_version."
-else
+if which black; then
     black --skip-string-normalization --line-length 100 --check .
+else
+    echo "Skipping black due to not being installed."
 fi

tox.ini

@@ -5,11 +5,11 @@ skipsdist = True
 [testenv]
 usedevelop = True
 deps = -rtest_requirements.txt
-whitelist_externals = bash
+whitelist_externals = sh
 commands =
     py.test --cov-report term-missing:skip-covered --cov=borgmatic --ignore=tests/end-to-end \
         tests []
-    bash scripts/black
+    sh scripts/black
     flake8 .
 
 [testenv:black]