Bump the fern MSRV to 1.38

The code did not change, but since the CI now uses a checked in
Cargo.lock.msrv of the new lockfile format introduced in Rust 1.38.

.github/workflows/rust.yml

@@ -117,7 +117,7 @@ jobs:
         with:
           profile: minimal
           # when updating this, also update rust-version in Cargo.toml
-          toolchain: 1.31.0
+          toolchain: 1.38.0
           override: true
       - run: cp Cargo.lock.msrv Cargo.lock
       - run: cargo build

Cargo.toml

@@ -5,7 +5,7 @@ version = "0.7.0"
 authors = ["David Ross <daboross@daboross.net>"]
 description = "Simple, efficient logging"
 edition = "2018"
-rust-version = "1.31"
+rust-version = "1.38.0"
 
 documentation = "https://docs.rs/fern/"
 repository = "https://github.com/daboross/fern"