travis: print cppcheck's output to stdout

This prevents Travis from believing that an error has occurred after
10 minutes without output. Capture error messages in a file and print
its content after cppcheck has finished.

Exit the script with cppcheck's exit code.

Thanks @turol for your help with this!

.travis.sh

@@ -1,8 +1,14 @@
 #!/bin/sh
-set -e
 if [ "$ANALYZE" = "true" ] ; then
-	cppcheck --error-exitcode=1 -j2 -UTESTING -Iopl -Isrc -Isrc/setup opl pcsound src textscreen > /dev/null
+	cppcheck --error-exitcode=1 -j2 -UTESTING -Iopl -Isrc -Isrc/setup opl pcsound src textscreen 2> stderr.txt
+	RET=$?
+	if [ -s stderr.txt ]
+	then
+		cat stderr.txt
+	fi
+	exit $RET
 else
+	set -e
 	./autogen.sh --enable-werror
 	make
 	make install DESTDIR=/tmp/whatever