Update references to codegangsta to urfave (new org name)

Also add notice to README

altsrc/flag.go

@@ -7,7 +7,7 @@ import (
 	"strconv"
 	"strings"
 
-	"github.com/codegangsta/cli"
+	"github.com/urfave/cli"
 )
 
 // FlagInputSourceExtension is an extension interface of cli.Flag that

altsrc/flag_test.go

@@ -8,7 +8,7 @@ import (
 	"testing"
 	"time"
 
-	"github.com/codegangsta/cli"
+	"github.com/urfave/cli"
 )
 
 type testApplyInputSource struct {

altsrc/input_source_context.go

@@ -3,7 +3,7 @@ package altsrc
 import (
 	"time"
 
-	"github.com/codegangsta/cli"
+	"github.com/urfave/cli"
 )
 
 // InputSourceContext is an interface used to allow

altsrc/map_input_source.go

@@ -6,7 +6,7 @@ import (
 	"strings"
 	"time"
 
-	"github.com/codegangsta/cli"
+	"github.com/urfave/cli"
 )
 
 // MapInputSource implements InputSourceContext to return

altsrc/yaml_command_test.go

@@ -11,7 +11,7 @@ import (
 	"os"
 	"testing"
 
-	"github.com/codegangsta/cli"
+	"github.com/urfave/cli"
 )
 
 func TestCommandYamlFileTest(t *testing.T) {

altsrc/yaml_file_loader.go

@@ -12,7 +12,7 @@ import (
 	"net/url"
 	"os"
 
-	"github.com/codegangsta/cli"
+	"github.com/urfave/cli"
 
 	"gopkg.in/yaml.v2"
 )