Travis CI: try to use 2 threads for Coverity Scan build

With GCC and Coverity together memory usage would be too high, but might work with Clang.

.travis.yml

@@ -58,7 +58,7 @@ addons:
       description: Build submitted via Travis CI
     notification_email: [email protected]
     build_command_prepend: "cov-configure --compiler clang-3.6 --comptype clangcc && cov-configure --comptype clangcxx --compiler clang++-3.6 && cmake -G Ninja .. -DCMAKE_BUILD_TYPE=DEBUG -DENABLE_LAUNCHER=1 -DENABLE_TEST=1"
-    build_command: ninja
+    build_command: ninja -j 2
     branch_pattern: coverity_scan
 
 notifications: