change ptypes call to timestamppb to fix linters

[aaron-prindle]

Seeing this error in CI:
full logs: https://app.travis-ci.com/github/GoogleContainerTools/skaffold/jobs/522665097

snippet

INFO [runner] linters took 2m39.517708463s with stages: goanalysis_metalinter: 2m15.060735761s, unused: 24.3748312s 
pkg/skaffold/event/event.go:557:14: SA1019: ptypes.TimestampNow is deprecated: Call the timestamppb.Now function instead. (staticcheck)
		Timestamp: ptypes.TimestampNow(),
		           ^
pkg/skaffold/event/event.go:572:10: SA1019: ptypes.TimestampNow is deprecated: Call the timestamppb.Now function instead. (staticcheck)
		ts:    ptypes.TimestampNow(),
		       ^
pkg/skaffold/event/event.go:28:2: SA1019: package github.com/golang/protobuf/ptypes is deprecated: Well-known types have specialized functionality directly injected into the generated packages for each message type. See the deprecation notice for each function for the suggested alternative. (staticcheck)
	"github.com/golang/protobuf/ptypes"
	^
pkg/skaffold/event/v2/event.go:379:20: SA1019: ptypes.TimestampNow is deprecated: Call the timestamppb.Now function instead. (staticcheck)
	event.Timestamp = ptypes.TimestampNow()
	                  ^
pkg/skaffold/event/v2/event.go:28:2: SA1019: package github.com/golang/protobuf/ptypes is deprecated: Well-known types have specialized functionality directly injected into the generated packages for each message type. See the deprecation notice for each function for the suggested alternative. (staticcheck)
	"github.com/golang/protobuf/ptypes"
	^

[gsquared94]

lgtm!

[codecov]

Codecov Report

Merging #6164 (e0f3356) into master (6b451a2) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6164      +/-   ##
==========================================
+ Coverage   69.32%   69.35%   +0.02%     
==========================================
  Files         481      481              
  Lines       18478    18478              
==========================================
+ Hits        12810    12815       +5     
+ Misses       4713     4711       -2     
+ Partials      955      952       -3     

Impacted Files	Coverage Δ
pkg/skaffold/event/event.go	91.10% <100.00%> (ø)
pkg/skaffold/event/v2/event.go	79.74% <100.00%> (ø)
pkg/skaffold/docker/parse.go	86.19% <0.00%> (-0.96%)	⬇️
pkg/skaffold/docker/image.go	79.53% <0.00%> (+1.39%)	⬆️
pkg/skaffold/util/tar.go	56.00% <0.00%> (+5.33%)	⬆️

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