Add release checkout script

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.1.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

Here are some examples:

➜  utils git:(release_checkout) ✗ ruby release_checkout.rb 3.0.4
Running checkout on OpenAPI Generator release 3.0.4
Checking homebrew forumla ... [ERROR]
> https://raw.githubusercontent.com/Homebrew/homebrew-core/master/Formula/openapi-generator.rb not yet updated with https://search.maven.org/remotecontent?filepath=org/openapitools/openapi-generator-cli/3.0.4/openapi-generator-cli-3.0.4.jar
Checking openapi-generator JAR ... [ERROR]
> http://central.maven.org/maven2/org/openapitools/openapi-generator/3.0.4/openapi-generator-3.0.4.jar not found
Checking openapi-generator-cli JAR ... [ERROR]
> http://central.maven.org/maven2/org/openapitools/openapi-generator-cli/3.0.4/openapi-generator-cli-3.0.4.jar not found
Checking openapi-generator-maven-plugin JAR ... [ERROR]
> http://central.maven.org/maven2/org/openapitools/openapi-generator-maven-plugin/3.0.4/openapi-generator-maven-plugin-3.0.4.jar not found
Checking openapi-generator-gradle-plugin JAR ... [ERROR]
> http://central.maven.org/maven2/org/openapitools/openapi-generator-gradle-plugin/3.0.4/openapi-generator-gradle-plugin-3.0.4.jar not found
Checking openapi-generator-online JAR ... [ERROR]
> http://central.maven.org/maven2/org/openapitools/openapi-generator-online/3.0.4/openapi-generator-online-3.0.4.jar not found
Checking openapi-generator-project pom.xml ... [ERROR]
> http://central.maven.org/maven2/org/openapitools/openapi-generator-project/3.0.4/openapi-generator-project-3.0.4.pom not found
Checking openapi-generator README.md ... [ERROR]
> '[3.0.4](https://github.com/OpenAPITools/openapi-generator/releases/tag/v3.0.4)' not found in README.md
> 'JAR location: `http://central.maven.org/maven2/org/openapitools/openapi-generator-cli/3.0.4/openapi-generator-cli-3.0.4.jar`' not found in README.md
> 'wget http://central.maven.org/maven2/org/openapitools/openapi-generator-cli/3.0.4/openapi-generator-cli-3.0.4.jar -O openapi-generator-cli.jar' not found in README.md
> 'Invoke-WebRequest -OutFile openapi-generator-cli.jar http://central.maven.org/maven2/org/openapitools/openapi-generator-cli/3.0.4/openapi-generator-cli-3.0.4.jar' not found in README.md
➜  utils git:(release_checkout) ✗ ruby release_checkout.rb 3.0.3
Running checkout on OpenAPI Generator release 3.0.3
Checking homebrew forumla ... [OK]
Checking openapi-generator JAR ... [OK]
Checking openapi-generator-cli JAR ... [OK]
Checking openapi-generator-maven-plugin JAR ... [OK]
Checking openapi-generator-gradle-plugin JAR ... [OK]
Checking openapi-generator-online JAR ... [OK]
Checking openapi-generator-project pom.xml ... [OK]
Checking openapi-generator README.md ... [OK]