modified after `embulk new java-filter test`

newGemPush.gradle

plugins {
    id "com.jfrog.bintray" version "1.1"
    id "com.github.jruby-gradle.base" version "0.1.5"
    id "java"
    id "checkstyle"
}
import com.github.jrubygradle.JRubyExec
repositories {
    mavenCentral()
    jcenter()
}
configurations {
    jrubyExec.extendsFrom gems
    provided
}

version = "0.1.0"

sourceCompatibility = 1.7
targetCompatibility = 1.7

dependencies {
    compile  "org.embulk:embulk-core:0.8.21"
    provided "org.embulk:embulk-core:0.8.21"
    // compile "YOUR_JAR_DEPENDENCY_GROUP:YOUR_JAR_DEPENDENCY_MODULE:YOUR_JAR_DEPENDENCY_VERSION"
    testCompile "junit:junit:4.+"

    // jruby
    gems "rubygems:bundler:1.12.3+"
    gems "rubygems:rake:10.+"
}

task classpath(type: Copy, dependsOn: ["jar"]) {
    doFirst { file("classpath").deleteDir() }
    from (configurations.runtime - configurations.provided + files(jar.archivePath))
    into "classpath"
}
clean { delete "classpath" }

checkstyle {
    configFile = file("${project.rootDir}/config/checkstyle/checkstyle.xml")
    toolVersion = '6.14.1'
}
checkstyleMain {
    configFile = file("${project.rootDir}/config/checkstyle/default.xml")
    ignoreFailures = true
}
checkstyleTest {
    configFile = file("${project.rootDir}/config/checkstyle/default.xml")
    ignoreFailures = true
}
task checkstyle(type: Checkstyle) {
    classpath = sourceSets.main.output + sourceSets.test.output
    source = sourceSets.main.allJava + sourceSets.test.allJava
}

task gem(type: JRubyExec, dependsOn: ["gemspec", "classpath"]) {
    jrubyArgs "-rrubygems/gem_runner", "-eGem::GemRunner.new.run(ARGV)", "build"
    script "${project.name}.gemspec"
    doLast { ant.move(file: "${project.name}-${project.version}.gem", todir: "pkg") }
}

task gemPush(type: JRubyExec, dependsOn: ["gem", "genRakefile", "genRakeCommand"]) {
    script "rake"
}

task genRakeCommand {
    ext.rakeCommand = file("rake")
    inputs.file "build.gradle"
    outputs.file rakeCommand
    doLast { rakeCommand.write($/
require "rake"
Rake.application.run
/$)
    }
}
clean { delete "rake" }

task genRakefile {
    ext.rakeFile = file("Rakefile")
    inputs.file "build.gradle"
    outputs.file rakeFile
    doLast { rakeFile.write($/
require "bundler/gem_tasks"
task default: "gemPushEmbulkJavaPlugin"

task "gemPushEmbulkJavaPlugin", [:remote] => ["release:guard_clean",
                                              "release:source_control_push"] do
  rubygem_push("pkg/" + "${project.name}-${project.version}.gem") if gem_push?
end
/$)
    }
}
clean { delete "Rakefile" }

task "package"(dependsOn: ["gemspec", "classpath"]) {
    doLast {
        println "> Build succeeded."
        println "> You can run embulk with '-L ${file(".").absolutePath}' argument."
    }
}

task gemspec {
    ext.gemspecFile = file("${project.name}.gemspec")
    inputs.file "build.gradle"
    outputs.file gemspecFile
    doLast { gemspecFile.write($/
Gem::Specification.new do |spec|
  spec.name          = "${project.name}"
  spec.version       = "${project.version}"
  spec.authors       = ["Civitaspo"]
  spec.summary       = %[Test filter plugin for Embulk]
  spec.description   = %[Test]
  spec.email         = ["civitaspo@gmail.com"]
  spec.licenses      = ["MIT"]
  # TODO set this: spec.homepage      = "https://github.com/civitaspo/embulk-filter-test"

  spec.files         = `git ls-files`.split("\n") + Dir["classpath/*.jar"]
  spec.test_files    = spec.files.grep(%r"^(test|spec)/")
  spec.require_paths = ["lib"]

  #spec.add_dependency 'YOUR_GEM_DEPENDENCY', ['~> YOUR_GEM_DEPENDENCY_VERSION']
  spec.add_development_dependency 'bundler', ['~> 1.0']
  spec.add_development_dependency 'rake', ['>= 10.0']
end
/$)
    }
}
clean { delete "${project.name}.gemspec" }

I have wanted to be the same behavior as https://github.com/bundler/bundler/blob/7c3d103932fe2b4c35a32b823722c3418130e7f1/lib/bundler/gem_helper.rb#L53-L69 ...