protoc-gen-swagger/genswagger does not build on go1.11 and earlier versions

[marcusljx]

Steps you follow to reproduce the error:

<!-- Example steps
1.  I grab my catapult
2.  I load it with lettuce
3.  Press the fire button
4.  It falls over
-->

1. Using go1.11
2. import github.com/grpc-ecosystem/grpc-gateway/protoc-gen-swagger/genswagger
3. use genswagger.New() in a main function
4. go build .
5. package fails to build because strings package does not have function ReplaceAll.

What did you expect to happen instead:

<!-- Example answer
1.  It would have rained lettuce from the sky bringing forth a cuddly army of bunnies we could
    play with
-->

go build should exit with exit code 0.

What's your theory on why it isn't working:

<!-- Example answer
Evil wizards are hoarding the bunnies and don't want to share. The wizards are casting 
lettuce protection spells so the catapult won't work.
-->

This line uses strings.ReplaceAll, which was only introduced in go1.12.

grpc-gateway/protoc-gen-swagger/genswagger/generator.go

Line 71 in e652ba0

return strings.ReplaceAll(strings.Title(k), "-", "_")

[marcusljx]

I would like to submit a small PR for this to allow for builds on earlier go versions too.