Don't need to install bundler manually

Author: hsbt

.github/workflows/macos.yml

@@ -8,8 +8,6 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - name: Install dependencies
-      run: |
-        gem install bundler --no-document
-        bundle install
+      run: bundle install
     - name: Run test
       run: rake && RUBYOPT=--enable-frozen_string_literal rake

.github/workflows/ubuntu.yml

@@ -15,8 +15,6 @@ jobs:
       with:
         ruby-version: ${{ matrix.ruby }}
     - name: Install dependencies
-      run: |
-        gem install bundler --no-document
-        bundle install
+      run: bundle install
     - name: Run test
       run: rake

.github/workflows/windows.yml

@@ -14,8 +14,6 @@ jobs:
       uses: ruby/setup-ruby@v1
       with:
         ruby-version: ${{ matrix.ruby }}
-    - name: Set up Bundler
-      run: gem install bundler --no-document --force
     - name: Install dependencies
       run: bundle install
     - name: Run test