make test: ImportError: No module named http

`make test` will fail with:

> $ make test [±master ●]
Starting unit tests...
Traceback (most recent call last):
  File "unit_tests/test_hooks.py", line 20, in <module>
    from hooks import hooks
  File "/home/guoqiao/charms/elasticsearch/hooks/hooks.py", line 3, in <module>
    import http
ImportError: No module named http
Makefile:12: recipe for target 'test' failed
make: *** [test] Error 1

A few issues here:
1. `http` is builtin lib in python3, but not in python2
2. according to Makefile, `make test` will run tests with python2
3. in `hooks.py`, the only place using `http` module is for a `HTTPStatus.OK` code(200), which could be easily avoided.