Install libstdc++ when testing on Travis
Install libstdc++ when testing on Travis

Found on [1]. Fixes the error [2] when running a build on Travis CI.

[1] https://github.com/nodegit/nodegit/issues/853
[2] Error: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version
`GLIBCXX_3.4.20' not found (required by
/home/travis/build/kimmobrunfeldt/git-hours/node_modules/nodegit/build/Release/nodegit.node)

kimmobrunfeldt/git-hours,mrPjer/git-hours