Pass -j4 to make to speedup libsymengine compilation

Author: isuruf

.travis.yml

@@ -109,6 +109,7 @@ matrix:
 install:
   - export PYTHON_SOURCE_DIR=`pwd`
   - export TEST_CPP="no"
+  - export MAKEFLAGS="-j2"
 
   # Setup travis for Python wrappers
   - source bin/install_travis.sh
@@ -124,6 +125,7 @@ install:
   # Build C++ library
   - cd $PYTHON_SOURCE_DIR/symengine-cpp
   - bin/test_travis.sh
+  - unset MAKEFLAGS
 
 script:
   # Build Python wrappers and test