Travis CI: disable output buffering using "go test -p 1"

Per default, "go test ./..." tests multiple packages in
parallel.

From "go help build":

	-p n
		the number of programs, such as build commands or
		test binaries, that can be run in parallel.
		The default is the number of CPUs available.

This seems rather innocent, but a consequence of tests
running in parallel is that the all the output of each test
is buffered until it completes. This prevents multiple tests
interleaving their output.

This also means that when a test hangs, we get no output at
all, which is pretty bad.

Disable parallelization and trade test time for better
debugability of hung tests.

Change-Id: I4c670b65baa2df3bef70bce622b830530a316ee7

.travis.yml

@@ -36,6 +36,6 @@ install:
 # as backup, triggering 1 minute later.
 script:
   - set -e # fail fast
-  - timeout -s QUIT -k 10s 90s go test -failfast -timeout 1m -v ./fs
-  - timeout -s QUIT -k 10s 6m  go test -failfast -timeout 5m -v ./...
+  - timeout -s QUIT -k 10s 90s go test -failfast -timeout 1m -p 1 -v ./fs
+  - timeout -s QUIT -k 10s 6m  go test -failfast -timeout 5m -p 1 -v ./...
   - set +e # restore