Try isolating jdk 9, download gradle through http to avoid openjdk6 key

failure

Author: akarnokd

.travis.yml

@@ -16,14 +16,16 @@ matrix:
   include:
     - jdk: openjdk6
     - jdk: oraclejdk8 # JDK 1.8.0_131-b11
-    - jdk: oraclejdk9 # JDK 9-ea+174 provided by Travis will be replaced by 9+177 in "before_install"
-      before_install:
-        - cd ~
-        - wget http://download.java.net/java/jdk9/archive/177/binaries/jdk-9+177_linux-x64_bin.tar.gz
-        - tar -xzf jdk-9+177_linux-x64_bin.tar.gz
-        - export JAVA_HOME=~/jdk-9
-        - PATH=$JAVA_HOME/bin:$PATH
-        - cd -
+    - os: linux
+      dist: trusty
+      jdk: oraclejdk9 # JDK 9-ea+174 provided by Travis will be replaced by 9+177 in "before_install"
+        before_install:
+          - cd ~
+          - wget http://download.java.net/java/jdk9/archive/177/binaries/jdk-9+177_linux-x64_bin.tar.gz
+          - tar -xzf jdk-9+177_linux-x64_bin.tar.gz
+          - export JAVA_HOME=~/jdk-9
+          - PATH=$JAVA_HOME/bin:$PATH
+          - cd -
 
 # Don't let Travis CI execute './gradlew assemble' by default
 install:

gradle/wrapper/gradle-wrapper.properties

@@ -3,4 +3,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-4.0.1-bin.zip
+distributionUrl=http\://services.gradle.org/distributions/gradle-4.0.1-bin.zip