disable publishing maven-metadata.xml checksums

Errors visible in https://oss.sonatype.org/#stagingRepositories under
Activity > release:

Artifact updating: Repository ='releases:Releases' does not allow updating
artifact='/com/github/dtreskunov/easyssl/maven-metadata.xml.sha512'

Artifact updating: Repository ='releases:Releases' does not allow updating
artifact='/com/github/dtreskunov/easyssl/maven-metadata.xml.sha256'

gradle/gradle#11308

.travis.yml

@@ -37,6 +37,7 @@ jobs:
         - ./gradlew dependencies
         - |
           ./gradlew \
+            -Dorg.gradle.internal.publish.checksums.insecure \
             -PnexusUsername=${SONATYPE_USERNAME} \
             -PnexusPassword=${SONATYPE_PASSWORD} \
             -Psigning.keyId=${GPG_KEY_ID} \