.travis.yml: specify "precise" Linux distro for JDK8 build

Travis's default Linux distro is now "xenial" !

.travis.yml

@@ -12,6 +12,7 @@ matrix:
   include:
   - os: linux
     jdk: oraclejdk8
+    dist: precise
     env: UPLOAD=true UPLOAD_NATIVE=true
   - os: linux
     jdk: openjdk11
@@ -82,4 +83,4 @@ deploy:
   # newest Android NDK
   # wget http://dl.google.com/android/ndk/android-ndk-r10c-linux-x86_64.bin -O ndk.bin
   # 7z x ndk.bin -y > /dev/null
-  # export ANDROID_NDK=`pwd`/android-ndk-r10c
+  # export ANDROID_NDK=`pwd`/android-ndk-r10c