Fix the test matrix

.travis.yml

@@ -11,9 +11,13 @@ osx_image: xcode7.3
 
 matrix:
   exclude:
+    # Disable all automatic entries in the test matrix
     - compiler: clang
+    - compiler: gcc
     - os: osx
+    - os: linux
   include:
+    # Explicitly add tests that we want to run
     - env: BUILD_DOXYGEN="yes"
       compiler: gcc
       os: linux
@@ -48,6 +52,14 @@ matrix:
         - doxygen --version
         - doxygen Doxyfile
         - echo "" > html/.nojekyll
+    - compiler: clang
+      os: osx
+    - compiler: gcc
+      os: osx
+    - compiler: clang
+      os: linux
+    - compiler: gcc
+      os: linux
 
 install:
   - cmake --version