More robust version check for Sage
Review Request #128926 - Created Sept. 16, 2016 and submitted
Version check for Sage is once again broken with the latest version. Instead of relying on parsing the sage banner, which is likely to break with changes in Sage output format, use the dedicated 'sage -v' command.
Correct version is detected again according to the console output. Note that the backend is still broken with latest Sage, but that's a separate issue.