Fix Travis CI

[Marcono1234]

Description

Travis CI fails for all builds. The relevant Travis log part is probably:

openjdk version "9.0.4"
OpenJDK Runtime Environment (build 9.0.4+11)
OpenJDK 64-Bit Server VM (build 9.0.4+11, mixed mode)
$ javac -J-Xmx32m -version
javac 9.0.4
3.54s$ mvn -f gson install -DskipTests=true
[INFO] Scanning for projects...
Downloading from google-maven-central: https://maven-central.storage-download.googleapis.com/repos/central/data/org/sonatype/oss/oss-parent/7/oss-parent-7.pom
Downloading from sonatype: https://oss.sonatype.org/content/repositories/releases/org/sonatype/oss/oss-parent/7/oss-parent-7.pom
Downloading from sonatype-apache: https://repository.apache.org/content/repositories/releases/org/sonatype/oss/oss-parent/7/oss-parent-7.pom
[ERROR] [ERROR] Some problems were encountered while processing the POMs:
[FATAL] Non-resolvable parent POM for com.google.code.gson:gson-parent:2.8.6-SNAPSHOT: Could not transfer artifact org.sonatype.oss:oss-parent:pom:7 from/to google-maven-central (https://maven-central.storage-download.googleapis.com/repos/central/data/): sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target and 'parent.relativePath' points at wrong local POM @ com.google.code.gson:gson-parent:2.8.6-SNAPSHOT, /home/travis/build/google/gson/pom.xml, line 6, column 11

It appears it tries all configured repositories and then fails:

1. maven-central.storage-download.googleapis.com
2. oss.sonatype.org (this just redirects to central!)
3. repository.apache.org

It looks like Maven mixes the exception messages, but the actual problem is:

sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

If we now compare the last successful build with the first known failing (there might have been earlier ones through pull requests, but Travis does not show them), we will see:

Successful	Failed
java version "9.0.1" Java(TM) SE Runtime Environment (build 9.0.1+11)	openjdk version "9.0.4" OpenJDK Runtime Environment (build 9.0.4+11)

And indeed there is JDK-8189357 which describes that the cacerts file for OpenJDK is empty.
To fix this a different truststore has to be specified, the certificates have to be imported, or you could switch to OpenJDK > 9 (see JEP 319), e.g. 11 which runs the build successfully as well and is also LTS.

[inder123]

Thanks. Can you send a PR that fixes the issue?

[Marcono1234]

Done, see #1607, that seems to fix the problem