Update MSRV to 1.38.0.

Author: jdm

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
 
     strategy:
       matrix:
-        rust: [1.36.0, nightly, beta, stable]
+        rust: [1.38.0, nightly, beta, stable]
 
     steps:
       - uses: actions/checkout@v2