Update common-requirements.txt

[jpmckinney]

No description provided.

[jpmckinney]

@Bjwebb Can you have a look at this failure? cc @robredpath

$ py.test -rs --tb=line
============================= test session starts ==============================
platform linux -- Python 3.6.3, pytest-2.8.7, py-1.4.31, pluggy-0.3.1
rootdir: /home/travis/build/open-contracting/standard, inifile: 
collected 18 items
test_docs.py ...F........s...
standard/schema/tests/test_schema_integrity.py ..
=========================== short test summary info ============================
SKIP [1] /home/travis/build/open-contracting/standard/test_docs.py:121: <Skipped instance>
=================================== FAILURES ===================================
/home/travis/build/open-contracting/standard/test_docs.py:80: assert None
=============== 1 failed, 16 passed, 1 skipped in 82.06 seconds ================

[jpmckinney]

Seems to have been an intermittent failure with the search server.