Fix Travis build (#800)

Summary:
Fix the `Werror` failures and update JUnit to 4.12 as we're making use of `Parameterized` features which aren't available in whichever version we have here.
Pull Request resolved: https://github.com/facebook/yoga/pull/800

Reviewed By: jknoxville

Differential Revision: D9132106

Pulled By: passy

fbshipit-source-id: 5e6a130ba65e4a1ebd1d9ec506fdb126ecd0e44a

.travis.yml

@@ -1,4 +1,12 @@
 language: java
+os: linux
+addons:
+  apt:
+    sources:
+      - llvm-toolchain-trusty-6.0
+      - ubuntu-toolchain-r-test
+    packages:
+      - clang-6.0
 
 env:
   - TARGET: website