Bump actions/cache from 1 to 2.1.5

Bumps [actions/cache](https://github.com/actions/cache) from 1 to 2.1.5.
- [Release notes](https://github.com/actions/cache/releases)
- [Commits](actions/cache@v1...v2.1.5)

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/elixir.yml

@@ -19,13 +19,13 @@ jobs:
         elixir: ['1.11', '1.10', '1.9']
     steps:
     - uses: actions/checkout@v2
-    - uses: actions/cache@v1
+    - uses: actions/cache@v2.1.5
       with:
         path: deps
         key: ${{ runner.os }}-mix-${{ hashFiles(format('{0}{1}', github.workspace, '/mix.lock')) }}
         restore-keys: |
           ${{ runner.os }}-mix-
-    - uses: actions/cache@v1
+    - uses: actions/cache@v2.1.5
       with:
         path: _build/ci
         key: ${{ runner.os }}-build-${{ hashFiles(format('{0}{1}', github.workspace, '/mix.lock')) }}
@@ -51,13 +51,13 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - uses: actions/cache@v1
+    - uses: actions/cache@v2.1.5
       with:
         path: deps
         key: ${{ runner.os }}-mix-${{ hashFiles(format('{0}{1}', github.workspace, '/mix.lock')) }}
         restore-keys: |
           ${{ runner.os }}-mix-
-    - uses: actions/cache@v1
+    - uses: actions/cache@v2.1.5
       with:
         path: _build/ci
         key: ${{ runner.os }}-build-${{ hashFiles(format('{0}{1}', github.workspace, '/mix.lock')) }}