added conditional include of lwjgl3 if java8 is supported; edited .travis.yml so that the project is compiled against multiple jdks

.travis.yml

@@ -3,6 +3,8 @@ sudo: false
 
 jdk:
   - oraclejdk8
+  - oraclejdk7
+  - openjdk7
 
 addons:
   ssh_known_hosts: updates.jmonkeyengine.org

settings.gradle

@@ -15,7 +15,9 @@ include 'jme3-desktop'
 include 'jme3-blender'
 include 'jme3-jogl'
 include 'jme3-lwjgl'
-include 'jme3-lwjgl3'
+if (JavaVersion.current().isJava8Compatible()) {
+    include 'jme3-lwjgl3'
+}
 
 // Other external dependencies
 include 'jme3-jbullet'