Update catch2 library to version 2.13.7 (#58)

The Mustache project fails to compile with newer glibc versions.
This is due to an incompatibility in the catch2 header, which was fixed starting 2.13.5 ([upstream discussion thread](catchorg/Catch2#2178)).

To fix the issue, I've updated the catch2 header to version 2.13.7 (the latest release at the time of writing this).