Fix clang build (#110)

Fix Clang build
diff --git a/.travis.yml b/.travis.yml
index 774bed7..731b74a 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -14,22 +14,20 @@
       - ubuntu-toolchain-r-test
 
 env:
-  global: CLANG_VERSION=8 GCC_VERSION=7
+  global: GCC_VERSION=7
 
 matrix:
   include:
-    - env: BUILD_TYPE=Release CC_COMPILER=clang-${CLANG_VERSION} CXX_COMPILER=clang++-${CLANG_VERSION}
+    - env: BUILD_TYPE=Release CC_COMPILER=clang CXX_COMPILER=clang++
       addons: &clang
         apt:
           packages:
             - *common_packages
-            - clang-8
+            - clang
           sources:
             - *common_sources
-            - sourceline: 'deb http://apt.llvm.org/jessie/ llvm-toolchain-jessie main'
-              key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
 
-    - env: BUILD_TYPE=Debug CC_COMPILER=clang-${CLANG_VERSION} CXX_COMPILER=clang++-${CLANG_VERSION}
+    - env: BUILD_TYPE=Debug CC_COMPILER=clang CXX_COMPILER=clang++
       addons: *clang
 
     - env: BUILD_TYPE=Release CC_COMPILER=gcc-${GCC_VERSION} CXX_COMPILER=g++-${GCC_VERSION}