issue #915: use "precise" environment only for openjdk7 build

.travis.yml

@@ -14,6 +14,7 @@ matrix:
     env: UPLOAD=true UPLOAD_NATIVE=true
   - os: linux
     jdk: openjdk7
+    dist: precise
   - os: osx
     osx_image: xcode9.3
     env: UPLOAD_NATIVE=true