Fix for "--tag" command-line argument

Fix for "--tag" command-line argument.  "expand_requirements_specifiers" requires
"tag" keyword argument.

@olegsinyavskiy Oleg, I tested this change with "--tag release-brainos1.0" and with the "--tag" option absent.  I don't know why Travis is failing to pass this.  The related branch in "eyerover" is "use_repo_tag_param", which does pass Travis C.I..