Merge tag '1.6.0' into develop

1.6.0
Preparation for new development cycle after release 1.6.0
Migration of gitflow_config fails for posix sh.

The substitution done in the loop isn't posix compliant.
Error messages when .gitflow_export doesn't exists.

When we check for the environment variables and the .gitflow_export files doesn't exists there are error messages. We need to check for these variables only when the file exists.

Closes: #104
Update dev version
Version bump 1.6.1-rc.1
Version bump 1.6.1
Merge branch 'release/1.6.1'