Slightly increase the number of parallel build jobs

Travis hosts typically 2 cores available, slightly bump
the number of parallel jobs to be (number of cores + 1)

Author: chrisr-diffblue

.travis.yml

@@ -252,8 +252,8 @@ install:
   - ccache --max-size=2G
   - make -C src minisat2-download
   - make -C src/ansi-c library_check
-  - make -C src "CXX=${COMPILER}" "CXXFLAGS=-Wall -Werror -pedantic -O2 -g ${EXTRA_CXXFLAGS}" -j2
-  - make -C src "CXX=${COMPILER}" "CXXFLAGS=-Wall -Werror -pedantic -O2 -g ${EXTRA_CXXFLAGS}" -j2 clobber.dir memory-models.dir
+  - make -C src "CXX=${COMPILER}" "CXXFLAGS=-Wall -Werror -pedantic -O2 -g ${EXTRA_CXXFLAGS}" -j3
+  - make -C src "CXX=${COMPILER}" "CXXFLAGS=-Wall -Werror -pedantic -O2 -g ${EXTRA_CXXFLAGS}" -j3 clobber.dir memory-models.dir
 
 script:
   - if [ -e bin/gcc ] ; then export PATH=$PWD/bin:$PATH ; fi ;