be nicer to travis OS X machines, remove unneeded 'make clean'

scripts/travis

@@ -5,8 +5,13 @@ set -e
 
 if [ "x$ARCH" = "xnative" ]; then
 	./configure --enable-libtls
-	make clean
+	if [ `uname` = "Darwin" ]; then
+		# OS X runs out of resources if we run 'make -j check'
+		make -j
+		make check
+	else
 		make -j distcheck
+	fi
 else
 	CPU=i686
 	if [ "x$ARCH" = "xmingw64" ]; then
@@ -25,6 +30,5 @@ else
 	fi
 
 	./configure --host=$CPU-w64-mingw32 --enable-libtls
-	make clean
 	make -j
 fi