[boost] Travis/Appveyor improvements for the superproject

Subject: [boost] Travis/Appveyor improvements for the superproject
From: Peter Dimov (lists_at_[hidden])
Date: 2017-09-20 13:33:06

Travis and Appveyor tests for the Boost superproject have been improved to 
automatically run the tests for the library mentioned in the commit message. 
For example, if the message is "Update iterator from develop", the 
corresponding CI job runs the tests of the Iterator library (among other 
things).

This is a bit redundant if the library itself has Travis/Appveyor set up, 
but keeping an eye on

https://ci.appveyor.com/project/boostorg/boost/history

and

https://travis-ci.org/boostorg/boost/builds

can't hurt.

For example, Appveyor says that Iterator fails a test on msvc-12.0 (Visual 
Studio 2013):

https://ci.appveyor.com/project/boostorg/boost/build/1.0.3744-develop

and Utility fails a test on msvc-14.0/msvc-14.1:

https://ci.appveyor.com/project/boostorg/boost/build/1.0.3765-develop