increase timeout to 30 minutes

nomad/client take very long and exceed 15m sometimes:

In https://travis-ci.org/hashicorp/nomad/jobs/452990197 :

```
panic: test timed out after 15m0s

goroutine 4739 [running]:
testing.(*M).startAlarm.func1()
	/home/travis/.gimme/versions/go1.11.2.linux.amd64/src/testing/testing.go:1296 +0xfd
....
goroutine 4665 [select]:
github.com/hashicorp/nomad/vendor/google.golang.org/grpc.newClientStream.func5(0xc0003dd500, 0xc000420120, 0x2b3f86295588, 0xc000496810)
	/home/travis/gopath/src/github.com/hashicorp/nomad/vendor/google.golang.org/grpc/stream.go:287 +0xd7
created by github.com/hashicorp/nomad/vendor/google.golang.org/grpc.newClientStream
	/home/travis/gopath/src/github.com/hashicorp/nomad/vendor/google.golang.org/grpc/stream.go:286 +0x842
FAIL	github.com/hashicorp/nomad/client/driver	900.036s
```

GNUmakefile

@@ -264,7 +264,7 @@ test-nomad: dev ## Run Nomad test suites
 	$(if $(ENABLE_RACE),GORACE="strip_path_prefix=$(GOPATH)/src") go test \
 		$(if $(ENABLE_RACE),-race) $(if $(VERBOSE),-v) \
 		-cover \
-		-timeout=900s \
+		-timeout=30m \
 		-tags="$(if $(HAS_LXC),lxc)" ./... $(if $(VERBOSE), >test.log ; echo $$? > exit-code)
 	@if [ $(VERBOSE) ] ; then \
 		bash -C "$(PROJECT_ROOT)/scripts/test_check.sh" ; \