Update CI config (#109)

Author: wojtekmach

.github/workflows/ci.yml

@@ -15,11 +15,11 @@ jobs:
       matrix:
         include:
           - elixir: 1.5.3
-            otp: 18.3.4.11
+            otp: 20.3.8.26
           - elixir: 1.6.6
-            otp: 19.3.6.13
+            otp: 20.3.8.26
           - elixir: 1.7.4
-            otp: 19.3.6.13
+            otp: 20.3.8.26
           - elixir: 1.8.2
             otp: 20.3.8.26
           - elixir: 1.9.4
@@ -28,7 +28,7 @@ jobs:
             otp: 23.0.3
     steps:
       - uses: actions/checkout@v2
-      - uses: actions/setup-elixir@v1
+      - uses: erlef/setup-elixir@v1
         with:
           otp-version: ${{matrix.otp}}
           elixir-version: ${{matrix.elixir}}
@@ -43,7 +43,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - uses: actions/setup-elixir@v1
+      - uses: erlef/setup-elixir@v1
         with:
           otp-version: 23.0.3
           elixir-version: 1.10.4
@@ -54,7 +54,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - uses: actions/setup-elixir@v1
+      - uses: erlef/setup-elixir@v1
         with:
           otp-version: 22.3.4.5
           elixir-version: 1.9.4 # not 1.10 as its --warnigs-as-errors has bugs https://github.com/elixir-lang/elixir/issues/10073