Build with c++11 flag

[springmeyer]

Unless c++14 is absolutely needed, we should stick to building/testing/supporting C++11.

I took a look at whether all the tests would build and pass in C++11. They do - the only thing requiring C++14 was the #include <mapbox/geometry.hpp> that indirectly pulled in mapbox/geometry/feature.hpp which pulls in std::experimental::optional from https://github.com/mapbox/geometry.hpp/blob/b0e41cc5635ff8d50e7e1edb73cadf1d2a7ddc83/include/mapbox/geometry/feature.hpp#L11.

Ideally, in my view geometry.hpp would be configured in such a way that std::experimental::optional is not forced on downstream users. Until this is fixed, simply using #include <mapbox/geometry/geometry.hpp> solves the problem elegantly and allows us to build in C++11 mode.

[artemp]

@springmeyer -

Unless c++14 is absolutely needed, we should stick to building/testing/supporting C++11.

We might or might not require c++14 support. My judgement is we will need it sooner rather than later. By sticking to c++11 we'll create user expectations which would be difficult to negotiate later.
There are of course issues with feature.hpp using experimental::optional and it should be possible to use just geometry.hpp without feature.hpp so lets fix them rather than starting with patching up and leaving unresolved issues for later.

[springmeyer]

Sounds good @artemp - Will close this now. Noting however that I see this PR also fixed a few places were we were missing includes. So I propose we fix that separately. I've ticketed at #20