Add CI tests for gcc5

[nmwsharp]

gcc5 loves to erroneously reject code with templates/tuples/initializers. However, a decent number of people still use it, as it is the default in Ubuntu 16.04. We should make sure our code passes on gcc5.

TODOs

• Add CI testing to catch gcc5 issues, similar to https://github.com/nmwsharp/polyscope/commit/d5ca9fab005c9c0feaf0a2758f524063e6b4a957
• Will probably also need to clean up some small failing code snippets