Upgrade to GitHub-native Dependabot

Author: dependabot-preview[bot]

.github/dependabot.yml

@@ -0,0 +1,19 @@
+version: 2
+updates:
+- package-ecosystem: mix
+  directory: "/"
+  schedule:
+    interval: daily
+    time: "04:00"
+  pull-request-branch-name:
+    separator: "-"
+  open-pull-requests-limit: 10
+  ignore:
+  - dependency-name: ex_doc
+    versions:
+    - 0.23.0
+    - 0.24.0
+    - 0.24.1
+  - dependency-name: excoveralls
+    versions:
+    - 0.13.4