Make the benchmarks group optional. (#1173)

It's for a contributor's smooth onboarding.
This commit fixes the failed installation of the benchmarks group
on Ruby >= 2.4 by the following error.

```
$ bundle install --without development
...
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
...
 current directory:
/opt/hostedtoolcache/Ruby/2.4.10/x64/lib/ruby/gems/2.4.0/gems/mysql-2.9.1/ext/mysql_api
make "DESTDIR="
compiling mysql.c
mysql.c: In function ‘stmt_bind_result’:
mysql.c:1320:74: error: ‘rb_cFixnum’ undeclared (first use in this function);
did you mean ‘rb_isalnum’?
else if (argv[i] == rb_cNumeric || argv[i] == rb_cInteger || argv[i] ==
rb_cFixnum)
^~~~~~~~~~
rb_isalnum
...
```

You can install the benchmarks group like this.

```
$ bundle install --with benchmarks
```

See https://bundler.io/man/gemfile.5.html#BLOCK-FORM-OF-SOURCE-GIT-PATH-GROUP-and-PLATFORMS .

.github/workflows/build.yml

@@ -58,7 +58,7 @@ jobs:
         with:
           ruby-version: ${{ matrix.ruby }}
       - run: ruby -v
-      - run: bundle install --without benchmarks development
+      - run: bundle install --without development
       - if: matrix.db != ''
         run: echo 'DB=${{ matrix.db }}' >> $GITHUB_ENV
       - run: sudo echo "127.0.0.1 mysql2gem.example.com" | sudo tee -a /etc/hosts

.travis.yml

@@ -1,7 +1,7 @@
 sudo: required
 dist: trusty
 language: ruby
-bundler_args: --without benchmarks development
+bundler_args: --without development
 # Pin Rubygems to a working version. Sometimes it breaks upstream. Update now and then.
 before_install:
   - gem --version

Gemfile

@@ -19,12 +19,13 @@ group :test do
   gem 'rubocop', '~> 0.50.0'
 end
 
-group :benchmarks do
+group :benchmarks, optional: true do
   gem 'activerecord', '>= 3.0'
   gem 'benchmark-ips'
   gem 'do_mysql'
   gem 'faker'
-  gem 'mysql'
+  # The installation of the mysql latest version 2.9.1 fails on Ruby >= 2.4.
+  gem 'mysql' if Gem::Version.new(RUBY_VERSION) < Gem::Version.new('2.4')
   gem 'sequel'
 end
 

appveyor.yml

@@ -7,7 +7,7 @@ install:
   - ruby --version
   - gem --version
   - bundler --version
-  - bundle install --without benchmarks --path vendor/bundle
+  - bundle install --path vendor/bundle
   - IF DEFINED MINGW_PACKAGE_PREFIX (ridk exec pacman -S --noconfirm --needed %MINGW_PACKAGE_PREFIX%-libmariadbclient)
 build_script:
   - bundle exec rake compile

ci/container.sh

@@ -3,7 +3,7 @@
 set -eux
 
 ruby -v
-bundle install --path vendor/bundle --without benchmarks development
+bundle install --path vendor/bundle --without development
 
 # Start mysqld service.
 bash ci/setup_container.sh