Add --no-dev flag to composer when installing dependencies and packaging Piwik releases
2.6.1 fixes  #6127
Merge branch 'master' of github.com:piwik/piwik

Conflicts:
	core/Version.php
Merge remote-tracking branch 'origin/master'
Debug failed build https://travis-ci.org/piwik/piwik/jobs/34646545