CI: Stop installing bundler explicitly

Author: mrkn

.github/workflows/ci.yml

@@ -58,8 +58,6 @@ jobs:
         python-version: ${{ matrix.python }}
         architecture: ${{ matrix.python_architecture }}
 
-    - run: gem install bundler
-
     - run: pip install --user numpy
 
     - run: bundle install
@@ -101,8 +99,6 @@ jobs:
       with:
         python-version: ${{ matrix.python }}
 
-    - run: gem install bundler
-
     - run: conda install numpy
 
     - run: bundle install