Update to newer requests-mock

Solves a possible issue with newer version of requests, plus some other
small-ish bugs.

Includes new functionality like better request history checking and some
nicer syntax.

Change-Id: I643ec2e1f136205445c532bb04216d8a76129a51
implement -s / --soft-update flag

This provides a new flag to update.py which allows for a "soft
update", which updates all the requirements found in g-r to the g-r
specified versions, but lets any unknown lines pass through as is,
without warning. This makes the tool usable for ecosystem projects
that want to test with g-r versions of the g-r specified dependencies,
but also want to be able to add their own dependencies above and beyond.

Change-Id: I1f195ef9ff1509659848e14ec9936ff6f66a6496
Merge "Update to newer requests-mock"
Merge "implement -s / --soft-update flag"