Install specific bundler version when minor and patch versions are given (#139)

Author: manuelmeurer

dist/index.js

@@ -229,7 +229,8 @@ async function installBundler(bundlerVersionInput, lockFile, platform, rubyPrefi
     console.log(`Using Bundler 1 shipped with ${engine}`)
   } else {
     const gem = path.join(rubyPrefix, 'bin', 'gem')
-    await exec.exec(gem, ['install', 'bundler', '-v', `~> ${bundlerVersion}`, '--no-document'])
+    const bundlerVersionConstraint = bundlerVersion.match(/^\d+\.\d+\.\d+/) ? bundlerVersion : `~> ${bundlerVersion}`
+    await exec.exec(gem, ['install', 'bundler', '-v', bundlerVersionConstraint, '--no-document'])
   }
 
   return bundlerVersion