Specify c++ flags on travis

Hi, I’m trying to get my addons tested on travis and appveyor.

The problem is, that one of the addon that I’ve made, require the use of std::regex_iterator. On Mac, this is not a problem, clang does not complains.
On linux, it needs g++ >= 4.9, and the flag -std=gnu++14.

The flag that get actually passed on my build, it is g++ -c -g3 -DDEBUG -Wall -std=c++11.

https://travis-ci.org/edap/ofxLSystemGrammar/jobs/208960422

Someone has an idea about how to change it?

Also, I do not know how much sense it has to make a configuration on the CI that includes this flag. Are the addons under linux supposed to work with the flag std=c++11?