Update shell test regex to support macOS development (#46)

Fixes #45 because the default shell on macOS returns
`/bin/sh: oof: command not found\n` which doesn't quite match the
regex provided.

After discussion with @iurisilvio, using a fairly lax regex which
just looks for "oof" followed by "not found" is good enough.

tests/test_shell.py

@@ -9,7 +9,7 @@ def test_shell_ok():
 
 
 def test_shell_error():
-    with pytest.raises(shell.MigrateCIShellException, match="/bin/sh: 1: oof: not found\n"):
+    with pytest.raises(shell.MigrateCIShellException, match=r"oof.+not found"):
         shell.exec("oof")