有一个Github上的开源Java项目，使用Gradle构建。现在需要发布到Maven中央仓库，其中需要配置一些中央仓库的用户、密码、签名等信息。这些都是私密信息，直接贴build脚本里面放在Github上肯定是不合适的。
Google了很久找到一个解决办法，在项目目录下创建gradle.properties文件。存放用户名密码信息，但实际值为空：

version=0.4.5-SNAPSHOT
signing.keyId=
signing.password=
signing.secretKeyRingFile=
ossrhUsername=
ossrhPassword=

现在只有一个问题，在使用travis-ci自动构建的时候，提示错误：

:signArchives FAILED
FAILURE: Build failed with an exception.
* What went wrong:
Could not evaluate onlyIf predicate for task ':signArchives'.
> Neither path nor baseDir may be null or empty string. path='' basedir='/home/travis/build/xyalan/marathon-cli'
* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.
BUILD FAILED

我的build脚本部分内容：

artifacts {
    archives javadocJar, sourcesJar
}

ext.isReleaseVersion = !version.endsWith("SNAPSHOT")
signing {
    required {
        boolean signed =  isReleaseVersion && gradle.taskGraph.hasTask("uploadArchives")
        print signed
        signed
    }
    sign configurations.archives
}

uploadArchives {
    repositories {
        mavenDeployer {
            beforeDeployment { MavenDeployment deployment -> signing.signPom(deployment) }

            repository(url: "https://oss.sonatype.org/service/local/staging/deploy/maven2/") {
                authentication(userName: ossrhUsername, password: ossrhPassword)
            }

            snapshotRepository(url: "https://oss.sonatype.org/content/repositories/snapshots/") {
                authentication(userName: ossrhUsername, password: ossrhPassword)
            }

            pom.project {
                name 'marathon-cli'
                packaging 'jar'
                // optionally artifactId can be defined here
                description 'A application used as an example on how to set up pushing  its components to the Central Repository.'
                url 'http://www.example.com/example-application'

                scm {
                    connection 'scm:git:git://github.com/xyalan/marathon-cli.git'
                    developerConnection 'scm:git:git@github.com:xyalan/marathon-cli.git'
                    url 'https://github.com/xyalan/marathon-cli'
                }

                licenses {
                    license {
                        name 'The Apache License, Version 2.0'
                        url 'http://www.apache.org/licenses/LICENSE-2.0.txt'
                    }
                }

                developers {
                    developer {
                        id 'alan'
                        name 'Alan Yang'
                        email 'alanlhy@gmail.com'
                    }
                }
            }
        }
    }

请问这个如何配置这个路径信息？