Meson improvements

[ximion]

I think mustache-d was one of the first D projects to have a Meson build definition, and it's showing its age - for example, by only allowing to build a static library, while meanwhile all D compilers can build shared libraries.

This PR contains one patch to modernize the Meson definition (and to add running tests to it), and another patch to test the Meson builds on the CI as well.

For the CI patch I am not sure yet whether it will succeed with GDC, since the GDC version on Travis is a bit old. If it fails, I will change the patch. Thanks for considering :-)

(I will push at least the non-static-lib change to Debian, because having only a static library causes problems with automatic package rebuilds when a new D compiler version is out)

[ximion]

As expected, GDC didn't like the change (it's version (4.8) is really old on Travis, 8.1 would work though). I excluded GDC from running Meson on the CI, so everything should be good now :-)