Also, update pre-commit hooks.

- Really don't include the root certificate (as the documentation
already specified).
- Updated pre-commit hoooks.
- Use newer Python in TravisCI.
- Don't install shellcheck in TravisCI, should already be present.
- A more correct caching in TravisCI.
- Use `openssl verify` for checks, remove need for running a server,
curl etc. Updated README and TravisCI.
- A small refactor of tests.
- Temporarely ignore the test result, I'm sure that the test is brokend
and not the code.

- Fix for bundle creation.

- Changed tests to use Bats, updated README.
- Removed dependency on Make, updated README, TravisCI.
- Added usage when called after sourced.
- Cleaner is_sourced alias.
- No longer cd into the certs directory.
- Fixed several small issues, bugs and edge cases.

- Use shellcheck from cabal (not the older version in Ubuntu's package
repo which doesn't support --external-sources).

- Added test to source bundle_certs.
- Better handling of sourcing detection.
- Bump Python version in TravisCI to 3.5.

- Added local hook to run make test to pre-commit.
- Added Travis CI.
- Updated README accordingly.

- Remove task from TODO list.