Update .travis.yml

Manually build examples and call ctest with a reduced number of CPUs to avoid Travis crashes.
diff --git a/.travis.yml b/.travis.yml
index 59212eb..45aba81 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -57,5 +57,6 @@
   - cmake .. -GNinja -DLIB_PROTO_MUTATOR_WITH_ASAN=ON -DLIB_PROTO_MUTATOR_DOWNLOAD_PROTOBUF=ON -DCMAKE_C_COMPILER=${CC_COMPILER} -DCMAKE_CXX_COMPILER=${CXX_COMPILER} -DCMAKE_BUILD_TYPE=${BUILD_TYPE}
 
 script:
-  - ninja 
-  - ninja -j8 check
+  - ninja
+  - ninja examples/all
+  - ctest -V -j8