Precompilation updates

* Require Elixir v1.9 or later
* Require precompilers to implement ElixirMake.Precompiler
* Remove precompiler_context callback as it is unused

Author: josevalim

Diff for: .github/workflows/ci.yml

@@ -8,19 +8,19 @@ on:
 
 jobs:
   test:
-    runs-on: ubuntu-16.04
+    runs-on: ubuntu-latest
     env:
       MIX_ENV: test
     strategy:
       fail-fast: false
       matrix:
         include:
           - pair:
-              elixir: 1.3.4
-              otp: 19.3.6.13
+              elixir: 1.9.4
+              otp: 20.3
           - pair:
-              elixir: 1.11.3
-              otp: 23.2.5
+              elixir: 1.13.4
+              otp: 24.3
             lint: lint
     steps:
       - uses: actions/checkout@v2