Update multiple Conan dependencies.

[ddalcino]

The fmt update is meant to address a bug reported in #81.

Note that spdlog/1.8.0 is dependent on fmt/7.0.3.

Related to #65: specific dependency versions supersede #65, but this does not replace the move to a separate file. Unless these dependencies are moved to a separate file, the process of bumping these dependencies cannot be easily automated, and PRs like this one will always be necessary to keep packages up to date.

[ddalcino]

Please note: Although the Travis and Appveyor builds of this are working just fine, the Github Actions workflow for this is failing: https://github.com/ddalcino/cpp_starter_project/runs/1227289080

The error message is something to do with a conditional in fmt/core.h with duplicate branches, caused by this line in src/main.cpp:44:41:

fmt::print("Hello, from {}\n", "{fmt}");

While the Travis builds use gcc-9, the Github Action build uses gcc-7. This is the only significant difference I can see between the working Travis build and the failing Github Action. I initially suspected that fmt/7.0.3 expects a newer compiler, but if you check fmt's CI build, they use g++-6, g++-8, clang-4, and clang-6 without any problem. I'm not sure what's going on here.

EDIT: The Github Actions workflow is working now: https://github.com/ddalcino/cpp_starter_project/actions/runs/494994408