venv rename renames the directory, but doesn't update the virutalenv scripts

[andrewcox]

http://pastebin.com/0LCDChaQ

Notice that 'pybrew venv rename ... ...' doesn't throw any errors, and a subsequent 'pybrew venv use' with the new venv name appears to work, but the resulting environment is broken (because the rename moved the venv scripts to a new directory, but did not change absolute paths in those scripts hard-coded to point to the old directory).