Update travis build to new container infrastructure (w/o sudo)

.travis.yml

@@ -5,12 +5,25 @@ compiler:
 env:
   - CXX11=1
   - CXX11=0
-before_install: ./util/travis/before_install.sh
+sudo: false
+addons:
+  apt:
+    sources:
+      - llvm-toolchain-precise-3.8
+      - ubuntu-toolchain-r-test
+    packages:
+      - cmake
+      - libgd2-noxpm-dev
+      - libsqlite3-dev
+      - p7zip
+      - g++-6
+      - clang-3.8
+before_install:
+  - # Nothing ever works correctly with precise, use a custom libleveldb build
+  - wget http://minetest.kitsunemimi.pw/libleveldb-1.18-ubuntu12.04.7z
+  - 7zr x -olibleveldb libleveldb-1.18-ubuntu12.04.7z
 script: ./util/travis/script.sh
 notifications:
   email: false
 matrix:
   fast_finish: true
-  exclude:
-    - env: CXX11=1
-      compiler: gcc