travis reduce number of compilation threads to prevent the gcc build from being killed

-the clang build seems fine past 4 threads

.travis.yml

@@ -20,7 +20,7 @@ install:
   - cd ~/pyston-build && ninja llvm_up
 
 script:
-  - cd ~/pyston-build && ninja lint check-format && ninja -j4 && ninja run_raytrace
+  - cd ~/pyston-build && ninja lint check-format && ninja -j2 && ninja run_raytrace
 
 os:
   - linux