Include nightly to github CI tests

Author: khvzak

.github/workflows/main.yml

@@ -106,7 +106,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-18.04, macos-latest, windows-latest]
-        rust: [stable]
+        rust: [stable, nightly]
         lua: [lua54, lua53, lua52, lua51, luajit]
         include:
         - os: ubuntu-18.04
@@ -141,7 +141,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-18.04, macos-latest]
-        rust: [stable]
+        rust: [stable, nightly]
         lua: [lua54, lua53, lua52, lua51, luajit]
         include:
         - os: ubuntu-18.04