Upgrading travis for clang 6

Signed-off-by: Álvaro Jurado <elbingmiss@gmail.com>

.travis.yml

@@ -3,9 +3,9 @@ sudo: required
 dist: trusty
 before_install:
   - sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
-  - sudo add-apt-repository 'deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-3.8 main' -y
   - sudo add-apt-repository 'deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-3.9 main' -y
   - sudo add-apt-repository 'deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-4.0 main' -y
+  - sudo add-apt-repository 'deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty-6.0 main' -y
   - sudo apt-get update
   - sudo apt-get install -y --force-yes $CC rc golang
 script: