Updates nupic.core to bff505ebfa3c107d78943f155ae79795e557d01d.
Remove references to nupic.core unit tests (not explicitly part of nupic)
Merge pull request #1554 from numenta-ci/core-update-sha

Updates nupic.core to latest built SHA.
Merge pull request #1557 from oxtopus/remove-nupic-core-tests

Remove references to nupic.core unit tests (not explicitly part of nupic)
Update README.md

"If you want develop" --> "If you want to develop"
Added coverage==3.7.1 to external/common/requirements.txt because pytest-cov==1.6 fails to supply the necessary dependency on the coverage package.
Merge pull request #1559 from vitaly-krugl/update-requirements

Added coverage==3.7.1 to external/common/requirements.txt because pytest...