Changing the dictionary in `get_unique_key()`

Closes #266 (closed)

The function get_unique_key() (https://gitlab.com/BuildStream/buildstream/blob/master/buildstream/plugins/elements/compose.py#L75) in buildstream/plugins/elements had an incorrectly initialised dictionary.

Initialising this dictionary correctly involved changing the integrate and include keys from one-tuples to just the value.

Once the dictionary was correctly initialised, the integration tests failed as the cache keys changed.

To fix this, the cache keys were updated by running ./tests/cachekey/update.py