Increase upper bound on size in excel export tests
Increase upper bound on size in excel export tests

Given this constraint in setup.py -- 'openpyxl>=1.6,<1.7' -- openpyxl 1.6.2 is getting installed during Travis CI builds. This version returns content slightly larger(4 bytes) than the previous upper bound. I increased the upper bound 10 bytes to account for variability in openpyxl versions.

murphyke/avocado,murphyke/avocado,murphyke/avocado,murphyke/avocado