Bump actions/cache from 2.1.5 to 2.1.6 (#100)

Bumps [actions/cache](https://github.com/actions/cache) from 2.1.5 to 2.1.6.
- [Release notes](https://github.com/actions/cache/releases)
- [Commits](https://github.com/actions/cache/compare/v2.1.5...v2.1.6)

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@v2.1.5
+      uses: actions/cache@v2.1.6
       with:
         path: ../Qt
         key: ${{ runner.os }}-QtCache-${{ matrix.qt-version }}