travis, don't run wml unit tests with gcc

To help prevent timeouts. Just running them with clang should be
fine.

.travis.yml

@@ -2,6 +2,15 @@ language: cpp
 compiler:
     - clang
     - gcc
+env:
+    - WML_TESTS=true
+    - WML_TESTS=false
+matrix:
+  exclude:
+    - compiler: clang 
+      env: WML_TESTS=false
+    - compiler: gcc 
+      env: WML_TESTS=true
 install:
     - sudo apt-get update -qq
     - sudo apt-get install -qq libboost-iostreams-dev libboost-program-options-dev libboost-regex-dev libboost-system-dev libboost-test-dev libcairo2-dev libfribidi-dev libpango1.0-dev libsdl-image1.2-dev libsdl-mixer1.2-dev libsdl-net1.2-dev libsdl-ttf2.0-dev
@@ -10,7 +19,7 @@ script:
     - "export DISPLAY=:99.0"
     - "sh -e /etc/init.d/xvfb start"
     - ./test_wrapper.sh
-    - ./run_wml_tests -t 15
+    - if [[ "$WML_TESTS" = true ]]; then ./run_wml_tests -t 15; fi
 after_failure:
     - ./travis_after_failure.sh
 notifications: