Need pandas 1.x.x if using new reportengine

Created by: tgiani

I was looking at #684 and trying to reproduce some of the examples of @wilsonmr . Running

template_text: |
 # Effective Exponents in Flavour Basis.
 ## Internal table
 {@ effective_exponents_table_internal @}

basis: flavour

pdf: 191015-mw-001

actions_:
 - report(main=True)

I m getting

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Traceback (most recent call last):
  File "/home/tommy/miniconda3/envs/nnpdf1/bin/validphys", line 11, in <module>
    load_entry_point('validphys', 'console_scripts', 'validphys')()
  File "/home/tommy/physics/nnpdfgit/nnpdf/validphys2/src/validphys/scripts/main.py", line 10, in main
    vp.main()
  File "/home/tommy/physics/nnpdfgit/nnpdf/validphys2/src/validphys/app.py", line 148, in main
    a.main()
  File "/home/tommy/physics/nnpdfgit/reportengine/src/reportengine/app.py", line 376, in main
    self.run()
  File "/home/tommy/physics/nnpdfgit/nnpdf/validphys2/src/validphys/app.py", line 144, in run
    super().run()
  File "/home/tommy/physics/nnpdfgit/reportengine/src/reportengine/app.py", line 361, in run
    rb.execute_sequential()
  File "/home/tommy/physics/nnpdfgit/reportengine/src/reportengine/resourcebuilder.py", line 164, in execute_sequential
    result = callspec.function(self.rootns, callspec.nsspec)
  File "/home/tommy/physics/nnpdfgit/reportengine/src/reportengine/report.py", line 426, in __call__
    expand_fuzzyspec=namespaces.expand_fuzzyspec,
  File "/home/tommy/miniconda3/envs/nnpdf1/lib/python3.7/site-packages/jinja2/environment.py", line 1090, in render
    self.environment.handle_exception()
  File "/home/tommy/miniconda3/envs/nnpdf1/lib/python3.7/site-packages/jinja2/environment.py", line 832, in handle_exception
    reraise(*rewrite_traceback_stack(source=source))
  File "/home/tommy/miniconda3/envs/nnpdf1/lib/python3.7/site-packages/jinja2/_compat.py", line 28, in reraise
    raise value.with_traceback(tb)
  File "<template>", line 3, in top-level template code
  File "/home/tommy/physics/nnpdfgit/reportengine/src/reportengine/report.py", line 422, in format_collect_fuzzyspec
    return as_markdown(res)
  File "/home/tommy/physics/nnpdfgit/reportengine/src/reportengine/report.py", line 398, in as_markdown
    return '\n'.join(as_markdown(elem) for elem in obj)
  File "/home/tommy/physics/nnpdfgit/reportengine/src/reportengine/report.py", line 398, in <genexpr>
    return '\n'.join(as_markdown(elem) for elem in obj)
  File "/home/tommy/physics/nnpdfgit/reportengine/src/reportengine/report.py", line 388, in as_markdown
    if hasattr(obj, 'as_markdown'):
  File "/home/tommy/physics/nnpdfgit/reportengine/src/reportengine/table.py", line 53, in as_markdown
    with pd.option_context('display.max_colwidth', None):
  File "/home/tommy/miniconda3/envs/nnpdf1/lib/python3.7/site-packages/pandas/core/config.py", line 406, in __enter__
    _set_option(pat, val, silent=True)
  File "/home/tommy/miniconda3/envs/nnpdf1/lib/python3.7/site-packages/pandas/core/config.py", line 123, in _set_option
    o.validator(v)
  File "/home/tommy/miniconda3/envs/nnpdf1/lib/python3.7/site-packages/pandas/core/config.py", line 764, in inner
    raise ValueError(msg.format(typ=_type))
ValueError: Value must have type '<class 'int'>'

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