* Added `bats-assert` for more detailed error, very useful locally and on Travis
* Moved `bats` to git submodule
Example:
```
not ok 52 checking opendkim: generator creates keys, tables and TrustedHosts
# (from function `assert_output' in file test/test_helper/bats-assert/src/assert.bash, line 239,
# in test file test/tests.bats, line 368)
# `assert_output 2' failed
#
# -- output differs --
# expected : 2
# actual : 4
# --
#
```
