Add bigdecimal to CI gemspec

Drop JRuby 9.1 CI.  Now that Sequel depends on bigdecimal, and the
only available bigdecimal gem has a required_ruby_version of 2.5+,
you cannot install the current version of Sequel on JRuby 9.1, and
therefore it doesn't make much sense testing it (even though it
currently works fine).

.ci.gemfile

@@ -62,6 +62,12 @@ else
   gem 'minitest', '>= 5.7.0'
 end
 
+if RUBY_VERSION < '2.4'
+  gem 'bigdecimal', '<1.3'
+else
+  gem 'bigdecimal'
+end
+
 # MRI Adapter Dependencies
 platforms :ruby do
   sequel_pg = RUBY_VERSION.split('.')[1].to_i.send(Time.now.yday.even? ? :even? : :odd?)
@@ -79,10 +85,6 @@ platforms :ruby do
   if sequel_pg
     gem 'sequel_pg', git: 'https://github.com/jeremyevans/sequel_pg', require: 'sequel'
   end
-
-  if RUBY_VERSION < '2.4'
-    gem 'bigdecimal', '<1.3'
-  end
 end
 
 # JRuby Adapter Dependencies

.github/workflows/ci.yml

@@ -29,7 +29,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ "1.9.3", "2.0.0", 2.1, 2.3, 2.4, 2.5, 2.6, 2.7, "3.0", 3.1, 3.2, jruby-9.1, jruby-9.2, jruby-9.3, jruby-9.4, truffleruby-head ]
+        ruby: [ "1.9.3", "2.0.0", 2.1, 2.3, 2.4, 2.5, 2.6, 2.7, "3.0", 3.1, 3.2, jruby-9.2, jruby-9.3, jruby-9.4, truffleruby-head ]
     name: ${{ matrix.ruby }}
     env:
       BUNDLE_GEMFILE: .ci.gemfile

README.rdoc

@@ -927,7 +927,7 @@ Sequel fully supports the currently supported versions of Ruby (MRI) and JRuby.
 support unsupported versions of Ruby or JRuby, but such support may be dropped in any
 minor version if keeping it becomes a support issue.  The minimum Ruby version
 required to run the current version of Sequel is 1.9.2, and the minimum JRuby version is
-9.0.0.0.
+9.2.0.0 (due to the bigdecimal dependency).
 
 == Maintainer