games/veloren: Unbreak build with Rust 1.52.0 (D30099)

error[E0635]: unknown feature `const_checked_int_methods`
  --> common/src/lib.rs:10:5
   |
10 |     const_checked_int_methods,
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^

http://package18.nyi.freebsd.org/data/122amd64-default-foo/2021-05-07_12h05m44s/logs/errors/veloren-0.9.0_1.log

PR:		255608