[CI] Run 'scons build' in parallel

Both Travis and Appveyor make at least two cores available

.travis.yml

@@ -22,5 +22,5 @@ before_script: |
   fi
   rm -f cantera.conf
 script:
-  - scons build VERBOSE=y python_package=full python3_package=n blas_lapack_libs=lapack,blas optimize=n
+  - scons build -j2 VERBOSE=y python_package=full python3_package=n blas_lapack_libs=lapack,blas optimize=n
   - scons test

appveyor.yml

@@ -7,7 +7,7 @@ install:
     C:\Python27-x64\Scripts\pip.exe install cython
 
 build_script:
-- cmd: C:\Python27-x64\scons build boost_inc_dir=C:\Libraries\boost_1_62_0 debug=n
+- cmd: C:\Python27-x64\scons build -j2 boost_inc_dir=C:\Libraries\boost_1_62_0 debug=n
 
 test_script:
 - ps: |