Importing bs4 in tests
I just got a bunch of test errors because I don't have bs4. I do have lxml. Haven't looked at this at all to see why the test isn't conditional. E.g.,

```
======================================================================
ERROR: test_string (pandas.io.tests.test_html.TestBs4ReadHtml)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/skipper/src/pandas-skipper/pandas/io/tests/test_html.py", line 221, in test_string
    df1 = self.run_read_html(data, '.*Water.*', infer_types=False)
  File "/home/skipper/src/pandas-skipper/pandas/io/tests/test_html.py", line 324, in run_read_html
    return _run_read_html(*args, **kwargs)
  File "/home/skipper/src/pandas-skipper/pandas/io/tests/test_html.py", line 31, in _run_read_html
    return read_html(*args, **kwargs)
  File "/home/skipper/src/pandas-skipper/pandas/io/html.py", line 701, in read_html
    for data in p.parse_raw_data()]
  File "/home/skipper/src/pandas-skipper/pandas/io/html.py", line 152, in parse_raw_data
    for rows in self.parse_rows()]
  File "/home/skipper/src/pandas-skipper/pandas/io/html.py", line 137, in parse_rows
    tables = self._parse_tables(self._build_doc(), self.match, self.attrs)
  File "/home/skipper/src/pandas-skipper/pandas/io/html.py", line 333, in _build_doc
    from bs4 import BeautifulSoup, SoupStrainer
ImportError: No module named bs4
```
