Upgrade ananconda-client from =1.6 to =1.7 when 1.7 bug is fixed

[martinholmer]

Recent development work on Package-Builder suggests that using anaconda-client 1.7.* packages causes an error, at least in a Linux/Python37 environment. This pull request is a reminder to periodically check anaconda-client development to see when Package-Builder can begin using the newest versions of anaconda-client. When Package-Builder is upgraded to use the newest anaconda-client version, Package-Builder will be revised so that the anaconda upload option --force is replaced by --skip-existing so that an upload does not overwrite existing packages.