Use the given Bundler 1.x version if supplied on old Rubies

* Fixes #327

Author: eregon

bundler.js

@@ -78,15 +78,18 @@ export async function installBundler(bundlerVersionInput, rubygemsInputSet, lock
 
   const floatVersion = common.floatVersion(rubyVersion)
 
-  if (engine === 'ruby' && floatVersion <= 2.2) {
-    console.log('Bundler 2 requires Ruby 2.3+, using Bundler 1 on Ruby <= 2.2')
-    bundlerVersion = '1'
-  } else if (engine === 'ruby' && /^2\.3\.[01]/.test(rubyVersion)) {
-    console.log('Ruby 2.3.0 and 2.3.1 have shipped with an old rubygems that only works with Bundler 1')
-    bundlerVersion = '1'
-  } else if (engine === 'jruby' && rubyVersion.startsWith('9.1')) { // JRuby 9.1 targets Ruby 2.3, treat it the same
-    console.log('JRuby 9.1 has a bug with Bundler 2 (https://github.com/ruby/setup-ruby/issues/108), using Bundler 1 instead on JRuby 9.1')
-    bundlerVersion = '1'
+  // Use Bundler 1 when we know Bundler 2 does not work
+  if (bundlerVersion.startsWith('2')) {
+    if (engine === 'ruby' && floatVersion <= 2.2) {
+      console.log('Bundler 2 requires Ruby 2.3+, using Bundler 1 on Ruby <= 2.2')
+      bundlerVersion = '1'
+    } else if (engine === 'ruby' && /^2\.3\.[01]/.test(rubyVersion)) {
+      console.log('Ruby 2.3.0 and 2.3.1 have shipped with an old rubygems that only works with Bundler 1')
+      bundlerVersion = '1'
+    } else if (engine === 'jruby' && rubyVersion.startsWith('9.1')) { // JRuby 9.1 targets Ruby 2.3, treat it the same
+      console.log('JRuby 9.1 has a bug with Bundler 2 (https://github.com/ruby/setup-ruby/issues/108), using Bundler 1 instead on JRuby 9.1')
+      bundlerVersion = '1'
+    }
   }
 
   // Workaround for truffleruby 22.0 + latest Bundler, use shipped Bundler instead: https://github.com/oracle/truffleruby/issues/2586