Switch to 4 threads for clang builds

8 jobs for clang_parallel
Status Name Job ID Coverage
  Test
passed clang-c++11-Qt4 #2296
Linux

00:18:16

passed clang-c++11-Qt5 #2300
Linux

00:11:45

passed clang-c++98-Qt4 #2298
Linux

00:08:00

passed clang-c++98-Qt5 #2302
Linux

00:09:49

passed gcc-c++11-Qt4 #2295
Linux

00:03:26

passed gcc-c++11-Qt5 #2299
Linux

00:03:18

passed gcc-c++98-Qt4 #2297
Linux

00:02:31

passed gcc-c++98-Qt5 #2301
Linux

00:02:48