Update scons to 3.1.2
Dropped Python 2.7 from the script. If the user still needs to build with Python 2.7 it's now forced to use python /usr/bin/scons.
Link to Changelog here.
Signed-off-by: Martin Reboredo <email@example.com>
Test Plan: Build mongo with it.
Reviewers: Triage Team, DataDrake
Reviewed By: Triage Team, DataDrake
Differential Revision: https://dev.getsol.us/D9051