update Travis CI script

.travis.yml

@@ -9,24 +9,26 @@ compiler:
   - clang
   - gcc
 
+dist: xenial
+
 addons:
   apt:
     sources:
       - ubuntu-toolchain-r-test
-      - llvm-toolchain-trusty-6.0
+      - llvm-toolchain-xenial-7
     packages:
       - g++-8
-      - clang-6.0
+      - clang-7
 
 before_install:
   - if [ "$CXX" = "g++" ]; then export CXX="g++-8"; fi
-  - if [ "$CXX" = "clang++" ]; then export CXX="clang++-6.0"; fi
+  - if [ "$CXX" = "clang++" ]; then export CXX="clang++-7"; fi
   - $CXX --version
 
 install:
-  - wget https://bitbucket.org/eigen/eigen/get/3.3.5.tar.gz
-  - tar xzf 3.3.5.tar.gz
-  - mv eigen-eigen-b3f3d4950030/Eigen include/
+  - wget https://bitbucket.org/eigen/eigen/get/3.3.7.tar.gz
+  - tar xzf 3.3.7.tar.gz
+  - mv eigen-eigen-323c052e1731/Eigen include/
 
 script:
   - cd test