Update go dep (#1560)

This fix updates go dep with `dep ensure --update` as well as the following:
- Removed github.com/ugorji/go restriction in Gopkg.toml (fixes  #1557)
- Added github.com/flynn/go-shlex in Makefile (neede by Caddy, maybe removed later)

This fix fixes #1557

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

vendor/github.com/aws/aws-sdk-go/internal/sdkrand/locked_source.go

@@ -0,0 +1,29 @@
+package sdkrand
+
+import (
+	"math/rand"
+	"sync"
+	"time"
+)
+
+// lockedSource is a thread-safe implementation of rand.Source
+type lockedSource struct {
+	lk  sync.Mutex
+	src rand.Source
+}
+
+func (r *lockedSource) Int63() (n int64) {
+	r.lk.Lock()
+	n = r.src.Int63()
+	r.lk.Unlock()
+	return
+}
+
+func (r *lockedSource) Seed(seed int64) {
+	r.lk.Lock()
+	r.src.Seed(seed)
+	r.lk.Unlock()
+}
+
+// SeededRand is a new RNG using a thread safe implementation of rand.Source
+var SeededRand = rand.New(&lockedSource{src: rand.NewSource(time.Now().UnixNano())})