add travis task : compile and test under C++17

[dota17]

From #2213, we could know that JSON_HAS_CPP_17 is false in all travis jobs.
Thus, we should add a task to compile and test under CXX17.

[coveralls]

Coverage remained the same at 100.0% when pulling 6a22533 on dota17:travis-task-cxx17 into eb7376b on nlohmann:develop.

[nlohmann]

We already have a test case above that sets CXXFLAGS=-std=c++2a. Maybe this can be reused.

[dota17]

Unfortunately, It seems that CXXFLAGS=-std=c++1z didn't work under c++17.

[karzhenkov]

CXXFLAGS=-std... is overriden by compile feature cxx_std_11 in top-level CMakeLists.txt (and by CMAKE_CXX_STANDARD, if it is present; see comment in #2117)

[dota17]

Reopening.
I've made a little investigation.
In GCC, the -std= command-line option have C++98, C++11, C++14, C++17, C++2a, so c++2a is different from c++17.
In Clang, the same option are the same, but use -std=c++1y in Clang 3.4 and earlier.

[dota17]

At the same time, I think we should update Ubuntu distribution in .travis.yml, because Precise and Trusty are EOL by Canonial.

[nlohmann]

At the same time, I think we should update Ubuntu distribution in .travis.yml, because Precise and Trusty are EOL by Canonial.

As long as they are supported by Travis, I am fine with the working setup. I am afraid that the CI breaks if we are moving to a newer version.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.