Use -j2 on the CI boxes

The travis-ci trusty container has two cores.

script/ci

@@ -6,5 +6,5 @@ set -e
 
 script/fetch-fixtures
 script/check-mallocs
-scan_build make
+scan_build make -j2
 script/test -b

script/test

@@ -88,9 +88,9 @@ else
 fi
 
 if [[ ! -z "$run_scan_build" ]]; then
-	scan_build make $target
+	scan_build make -j2 $target
 else
-	make $target
+	make -j2 $target
 fi
 args=${args:-""}