build-lambda-zip: pass pointer to cli.StringFlag instead of value

[bmoffatt]

Issue #, if available:

#248

Description of changes:

Without this change, the following compile error is seen when installing build-lambda-zip using go get

$ go get -u github.com/bmoffatt/aws-lambda-go/cmd/build-lambda-zip
# github.com/bmoffatt/aws-lambda-go/cmd/build-lambda-zip
go/src/github.com/bmoffatt/aws-lambda-go/cmd/build-lambda-zip/main.go:19:17: cannot use cli.StringFlag literal (type cli.StringFlag) as type cli.Flag in array or slice literal:
	cli.StringFlag does not implement cli.Flag (Apply method has pointer receiver)

after pushing this change to my fork go get -u github.com/bmoffatt/aws-lambda-go/cmd/build-lambda-zip works as expected

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-io]

Codecov Report

Merging #249 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #249   +/-   ##
=======================================
  Coverage   74.59%   74.59%           
=======================================
  Files          20       20           
  Lines         681      681           
=======================================
  Hits          508      508           
  Misses        128      128           
  Partials       45       45

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