Update test deps

[hardbyte]

Pinning an old version of coverage avoids a coverage bug we are running into - see this failing travis build on develop with travis on MACOS on Python3.6.0 where after the tests are complete we get an internal error:

INTERNALERROR> Traceback (most recent call last):

INTERNALERROR>   File "/Users/travis/virtualenv/python3.6-dev/lib/python3.6/site-packages/coverage/sqldata.py", line 1032, in execute

INTERNALERROR>     return self.con.execute(sql, parameters)

INTERNALERROR> sqlite3.OperationalError: Safety level may not be changed inside a transaction

INTERNALERROR> 

Appears to be nedbat/coveragepy#703

[codecov]

Codecov Report

Merging #745 into develop will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #745      +/-   ##
===========================================
+ Coverage    69.51%   69.52%   +0.01%     
===========================================
  Files           70       70              
  Lines         6485     6485              
===========================================
+ Hits          4508     4509       +1     
+ Misses        1977     1976       -1