[boost] Travis CI again.

Sorry, but I need help, and simply can not see what's going wrong with: https://github.com/boostorg/config/pull/286.  Travis doesn't even seem to be building the same code as has been pushed? And until I can get CI functioning again here, and in other libraries, nothing gets merged to master :( Sorry if I'm being dense, and thanks in advance, John. --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus