set simple docker test to only be executed in linux

.travis.yml

@@ -66,7 +66,7 @@ notifications:
 install:
   # Build SimulationCraft
   - if [ "$TRAVIS_OS_NAME" == "linux" ]; then make CXX=g++-5 DEBUG=1 -C engine -j 2; fi
-  - if [ "$TRAVIS_OS_NAME" == "linux" ]; then docker build --build-arg THREADS=2 --build-arg NONETWORKING=1 -t simulationcraft . ; fi
+  - if [ "$TRAVIS_OS_NAME" == "linux" ]; then docker build --no-cache --build-arg THREADS=2 --build-arg NONETWORKING=1 -t simulationcraft . ; fi
   - if [ "$TRAVIS_OS_NAME" == "osx"   ]; then make DEBUG=1 SANITIZE=1 -C engine -j 2; fi
 
 script:
@@ -82,4 +82,5 @@ script:
   - if [ "$TRAVIS_OS_NAME" == "linux" ]; then valgrind --show-reachable=yes --leak-check=summary ./engine/simc PR_Raid.simc iterations=5 threads=1 fight_style=HecticAddCleave output=/dev/null cleanup_threads=1; fi
   - if [ "$TRAVIS_OS_NAME" == "linux" ]; then valgrind --show-reachable=yes --leak-check=summary ./engine/simc PR_Raid.simc iterations=5 threads=1 fight_style=BeastLord output=/dev/null cleanup_threads=1; fi
 
-  - docker run simulationcraft ./simc spell_query=spell.name=frost_shock
+  # simple docker test
+  - if [ "$TRAVIS_OS_NAME" == "linux" ]; then docker run simulationcraft ./simc spell_query=spell.name=frost_shock; fi