Used gradle wrapper to retrieve gradle with specific version in CI en…

…vironment.

.travis.yml

@@ -12,9 +12,10 @@ before_install:
 - echo "security.provider.1=org.bouncycastle.jce.provider.BouncyCastleProvider" | sudo tee -a /etc/java-7-openjdk/security/java.security
 - sudo add-apt-repository ppa:cwchien/gradle -y
 - sudo apt-get update -q
-- sudo apt-get install gradle=5.1 -y  # Restrict Gradle to 5.1 for JDK7 compatibility
-- export PATH=/usr/bin:$PATH  # Use gradle from /usr/bin
 - gradle --version
+- gradle wrapper
+- ./gradlew wrapper --gradle-version 5.1  # Restrict Gradle to 5.1 for JDK7 compatibility
+- export PATH=/usr/bin:$PATH  # Use gradle from /usr/bin
 
 before_script:
 - "export DISPLAY=:99.0"