[CI] run travis on trusty & xenial

.travis.yml

 language: cpp
 
-os:
-  - linux
-  - osx
-
-compiler:
-  - gcc
-  - clang
-
 env:
   - BUILD_TYPE=Debug
   - BUILD_TYPE=Release
 
 matrix:
-  exclude:
-    - os: osx
+  include:
+    - dist: trusty
       compiler: gcc
+    - dist: xenial
+      compiler: gcc
+    - dist: trusty
+      compiler: clang
+    - dist: xenial
+      compiler: clang
+    - os: osx
+      compiler: clang
 
 install:
   # Install dependencies for FCL
@@ -25,7 +25,7 @@ install:
 script:
   # Create build directory
   - mkdir build
-  - cd build 
+  - cd build
 
   # Configure
   - cmake -DCMAKE_BUILD_TYPE=$BUILD_TYPE -DCMAKE_CXX_FLAGS=-w ..
@@ -39,4 +39,3 @@ script:
   # Make sure we can install and uninstall with no issues
   - sudo make -j4 install
   - sudo make -j4 uninstall
-