Bump actions/cache from 2 to 2.1.5

Bumps [actions/cache](https://github.com/actions/cache) from 2 to 2.1.5.
- [Release notes](https://github.com/actions/cache/releases)
- [Commits](https://github.com/actions/cache/compare/v2...v2.1.5)

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

Closes #2

Change-Id: I9ff58caaaa291bcc6dd6b2d6d22b03e63f720524
diff --git a/.github/workflows/pythonpackage.yml b/.github/workflows/pythonpackage.yml
index dedeedd..2d51cb5 100644
--- a/.github/workflows/pythonpackage.yml
+++ b/.github/workflows/pythonpackage.yml
@@ -32,7 +32,7 @@
       with:
         python-version: ${{ matrix.python-version }}
     - name: Cache pip
-      uses: actions/cache@v2
+      uses: actions/cache@v2.1.5
       with:
         # This path is specific to Ubuntu
         path: ~/.cache/pip
@@ -56,13 +56,13 @@
     - name: Install R dependencies
       run: Rscript -e "install.packages('RKorAPClient')"
     - name: Cache R packages on Linux
-      uses: actions/cache@v1
+      uses: actions/cache@v2.1.5
       with:
         path: /usr/local/lib/R/site-library
         key: ${{ runner.os }}-r-${{ matrix.config.r-version }}-${{ hashFiles('DESCRIPTION') }}
       if: runner.os == 'Linux'
     - name: Cache R packages
-      uses: actions/cache@v1
+      uses: actions/cache@v2.1.5
       with:
         path: ${{ env.R_LIBS_USER }}
         key: ${{ runner.os }}-r-${{ matrix.config.r-version }}-${{ hashFiles('DESCRIPTION') }}