use build type set in .travis.yml

The BUILD_TYPE variable of .travis.yml was ignored up to now.
diff --git a/travis.sh b/travis.sh
index 1b23a3d..819e366 100755
--- a/travis.sh
+++ b/travis.sh
@@ -9,6 +9,7 @@
       -Dgtest_build_tests=ON \
       -Dgmock_build_tests=ON \
       -DCMAKE_CXX_FLAGS=$CXX_FLAGS \
+      -DCMAKE_BUILD_TYPE=$BUILD_TYPE \
       ..
 make
 CTEST_OUTPUT_ON_FAILURE=1 make test