lxml missing from requirements_dev.txt
#### Problem description

Tests fail to pass after installing a clean pandas development environment, .because the lxml package is missing.

Based on the instruction at https://pandas.pydata.org/pandas-docs/stable/contributing.html, to set up the environment and run the tests I followed:

> conda create -n pandas_dev --file ci/requirements_dev.txt
> source activate pandas_dev
> python setup.py build_ext --inplace
> pytest pandas

And this is the result:

<details>
================================================================================================================= FAILURES =================================================================================================================
______________________________________________________________________________________________________ test_importcheck_thread_safety ______________________________________________________________________________________________________

    @pytest.mark.slow
    def test_importcheck_thread_safety():
        # see gh-16928

        # force import check by reinitalising global vars in html.py
        reload(pandas.io.html)

        filename = os.path.join(DATA_PATH, 'valid_markup.html')
        helper_thread1 = ErrorThread(target=read_html, args=(filename,))
        helper_thread2 = ErrorThread(target=read_html, args=(filename,))

        helper_thread1.start()
        helper_thread2.start()

        while helper_thread1.is_alive() or helper_thread2.is_alive():
            pass
>       assert None is helper_thread1.err is helper_thread2.err
E       AssertionError: assert None is ImportError('lxml not found, please install it',)
E        +  where ImportError('lxml not found, please install it',) = <ErrorThread(Thread-1, stopped 123145411235840)>.err

pandas/tests/io/test_html.py:967: AssertionError
============================================================================================================= warnings summary =============================================================================================================
pandas/tests/dtypes/test_missing.py::test_array_equivalent_compat
  /Users/marcgarcia/anaconda3/envs/pandas_dev/lib/python3.6/site-packages/numpy/core/numeric.py:2604: FutureWarning: elementwise == comparison failed and returning scalar instead; this will raise an error or perform elementwise comparison in the future.
    return bool(asarray(a1 == a2).all())

-- Docs: http://doc.pytest.org/en/latest/warnings.html
======================================================================== 1 failed, 12619 passed, 1940 skipped, 11 xfailed, 1 xpassed, 1 warnings in 733.55 seconds =========================================================================
</details>

#### Expected Output

After installing lxml, all tests pass.

> conda install lxml
> pytest pandas

<details>
============================================================================================================= warnings summary =============================================================================================================
pandas/tests/dtypes/test_missing.py::test_array_equivalent_compat
  /Users/marcgarcia/anaconda3/envs/pandas_dev/lib/python3.6/site-packages/numpy/core/numeric.py:2604: FutureWarning: elementwise == comparison failed and returning scalar instead; this will raise an error or perform elementwise comparison in the future.
    return bool(asarray(a1 == a2).all())

-- Docs: http://doc.pytest.org/en/latest/warnings.html
============================================================================= 12627 passed, 1933 skipped, 11 xfailed, 1 xpassed, 1 warnings in 724.88 seconds ==============================================================================
</details>

#### Output of ``pd.show_versions()``

<details>

>>> pandas.show_versions()

INSTALLED VERSIONS
------------------
commit: bf5b08980526e36e03bc8f4637b8028e0e2d6145
python: 3.6.2.final.0
python-bits: 64
OS: Darwin
OS-release: 16.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

pandas: 0.21.0.dev+558.gbf5b08980.dirty
pytest: 3.2.2
pip: 9.0.1
setuptools: 36.3.0
Cython: 0.27
numpy: 1.13.3
scipy: None
pyarrow: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
