ci: use two parallel build jobs on Travis-CI

Change-Id: I734c5826f37e1027484ef9723933e8954dba00a8
diff --git a/.jenkins.d/01-ndn-cxx.sh b/.jenkins.d/01-ndn-cxx.sh
index 0225e2d..c9e2a63 100755
--- a/.jenkins.d/01-ndn-cxx.sh
+++ b/.jenkins.d/01-ndn-cxx.sh
@@ -42,8 +42,8 @@
 
 pushd ndn-cxx >/dev/null
 
-./waf configure -j1 --color=yes --enable-shared --disable-static --without-osx-keychain
-./waf -j1 --color=yes
+./waf configure --color=yes --enable-shared --disable-static --without-osx-keychain
+./waf build --color=yes -j${WAF_JOBS:-1}
 sudo env "PATH=$PATH" ./waf install --color=yes
 
 popd >/dev/null