Add the default timeout for http request (#35)

This PR sets the default http timout to 60 seconds.
diff --git a/.travis.yml b/.travis.yml
index a6085a6..71fc279 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -6,6 +6,7 @@
     include:
         - os: linux
           sudo: required
+          group: deprecated-2017Q3
           go: 1.8
           services: docker
 
diff --git a/whisk/client.go b/whisk/client.go
index 985bc75..c8cb311 100644
--- a/whisk/client.go
+++ b/whisk/client.go
@@ -31,6 +31,7 @@
     "reflect"
     "github.com/apache/incubator-openwhisk-client-go/wski18n"
     "strings"
+    "time"
 )
 
 const (
@@ -46,6 +47,7 @@
     DoNotProcessTimeOut = false
     ExitWithErrorOnTimeout = true
     ExitWithSuccessOnTimeout = false
+    DEFAULT_HTTP_TIMEOUT = 30
 )
 
 type Client struct {
@@ -91,6 +93,12 @@
         config = config_input
     }
 
+    if httpClient == nil {
+        httpClient = &http.Client{
+            Timeout: time.Second * DEFAULT_HTTP_TIMEOUT,
+        }
+    }
+
     // Disable certificate checking in the dev environment if in insecure mode
     if config.Insecure {
         Debug(DbgInfo, "Disabling certificate checking.\n")
@@ -108,15 +116,11 @@
             }
         }
 
-        http.DefaultClient.Transport = &http.Transport{
+        httpClient.Transport = &http.Transport{
             TLSClientConfig: tlsConfig,
         }
     }
 
-    if httpClient == nil {
-        httpClient = http.DefaultClient
-    }
-
     var err error
     var errStr = ""
     if len(config.Host) == 0 {