Fix clang installation on Travis

.travis.yml

@@ -84,12 +84,12 @@ matrix:
       env: SCRIPT=test _CC=clang-4.0 _CXX=clang++-4.0
     - addons:
         apt:
-          sources: ['ubuntu-toolchain-r-test','llvm-toolchain-trusty-5.0']
+          sources: ['ubuntu-toolchain-r-test']
           packages: ['clang-5.0']
       env: SCRIPT=test _CC=clang-5.0 _CXX=clang++-5.0
     - addons:
         apt:
-          sources: ['ubuntu-toolchain-r-test','llvm-toolchain-trusty-6.0']
+          sources: ['ubuntu-toolchain-r-test']
           packages: ['clang-6.0']
       env: SCRIPT=test _CC=clang-6.0 _CXX=clang++-6.0
     - addons:
@@ -121,7 +121,7 @@ matrix:
     - env: SCRIPT=platformio BOARD=esp01
     - addons:
         apt:
-          sources: ['ubuntu-toolchain-r-test','llvm-toolchain-trusty-6.0']
+          sources: ['ubuntu-toolchain-r-test']
           packages: ['clang-6.0','llvm-6.0']
       env: SCRIPT=fuzz CLANG=6.0
 cache: