[ci] Reactivate the Linux compilation.

Signed-off-by: Stéphane Galland <galland@arakhne.org>

.travis.yml

@@ -9,25 +9,25 @@ matrix:
   fast_finish: true
   include:
 
-#    - os: linux
-#      jdk: oraclejdk8
-#      addons:
-#        apt:
-#          packages:
-#          - xvfb
-#          - lib32z1
-#          - lib32ncurses5
-#          - genisoimage
-#      before_install:
-#      - "sh -e ./build-tools/scripts/travis-download-maven.sh"
-#      - export M2_HOME=$HOME/.binaries/apache-maven-3.3.9
-#      - export PATH=$M2_HOME/bin:$PATH
-#      - mvn -Xmx32m --version
-#      - "export DISPLAY=:99.0"
-#      - "sh -e /etc/init.d/xvfb start"
-#      - sleep 3 # give xvfb some time to start
-#      - echo "MAVEN_OPTS=\"$MAVEN_OPTS -XX:+CMSClassUnloadingEnabled -Xmx3g\"" > $HOME/.mavenrc
-#      env: DISPLAY=:99.0
+    - os: linux
+      jdk: oraclejdk8
+      addons:
+        apt:
+          packages:
+          - xvfb
+          - lib32z1
+          - lib32ncurses5
+          - genisoimage
+      before_install:
+      - "sh -e ./build-tools/scripts/travis-download-maven.sh"
+      - export M2_HOME=$HOME/.binaries/apache-maven-3.3.9
+      - export PATH=$M2_HOME/bin:$PATH
+      - mvn -Xmx32m --version
+      - "export DISPLAY=:99.0"
+      - "sh -e /etc/init.d/xvfb start"
+      - sleep 3 # give xvfb some time to start
+      - echo "MAVEN_OPTS=\"$MAVEN_OPTS -XX:+CMSClassUnloadingEnabled -Xmx3g\"" > $HOME/.mavenrc
+      env: DISPLAY=:99.0
 
     - os: osx
       osx_image: xcode8.2 # see https://docs.travis-ci.com/user/osx-ci-environment/