Bump actions/cache from v2.1.4 to v2.1.5 (#91)

Bumps [actions/cache](https://github.com/actions/cache) from v2.1.4 to v2.1.5.
- [Release notes](https://github.com/actions/cache/releases)
- [Commits](https://github.com/actions/cache/compare/v2.1.4...1a9e2138d905efd099035b49d8b7a3888c653ca8)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/ci.yaml

@@ -43,7 +43,7 @@ jobs:
     - name: Cache Qt
       if: matrix.os == 'ubuntu-16.04' # We might want to do this for both mac and linux
       id: cache-qt
-      uses: actions/cache@v2.1.4
+      uses: actions/cache@v2.1.5
       with:
         path: ../Qt
         key: ${{ runner.os }}-QtCache-${{ matrix.qt-version }}