Fixed a few regex warnings

[azmeuk]

Raised by pytest:

readability/readability.py:36
  /home/azmeuk/dev/python-readability/readability/readability.py:36: DeprecationWarning: invalid escape sequence \/
    'videoRe': re.compile('https?:\/\/(www\.)?(youtube|vimeo)\.com', re.I),

readability/readability.py:60
  /home/azmeuk/dev/python-readability/readability/readability.py:60: DeprecationWarning: invalid escape sequence \s
    text = re.sub('\s*\n\s*', '\n', text)

readability/readability.py:274
  /home/azmeuk/dev/python-readability/readability/readability.py:274: DeprecationWarning: invalid escape sequence \.
    and re.search('\.( |$)', node_content):

readability/encoding.py:54
  /home/azmeuk/dev/python-readability/readability/encoding.py:54: DeprecationWarning: invalid escape sequence \s
    text = re.sub(b'(\s*</?[^>]*>)+\s*', b' ', page).strip()

readability/debug.py:44
  /home/azmeuk/dev/python-readability/readability/debug.py:44: DeprecationWarning: invalid escape sequence \s
    RE_COLLAPSE_WHITESPACES = re.compile('\s+', re.U)

-- Docs: https://docs.pytest.org/en/latest/warnings.html