Update to use go modules (#671)

This includes updates to CI config, vendor files, etc.
I've hard-coded the k8s depedency at 1.13.4 to keep it inline with what
we currently have and to make sure a another run of `go mod tidy`
doesn't accidentally update it to an unexpected version.

Thanks to hectorj2f for carrying this along.

vendor/golang.org/x/text/unicode/bidi/bracket.go

@@ -12,7 +12,7 @@ import (
 
 // This file contains a port of the reference implementation of the
 // Bidi Parentheses Algorithm:
-// http://www.unicode.org/Public/PROGRAMS/BidiReferenceJava/BidiPBAReference.java
+// https://www.unicode.org/Public/PROGRAMS/BidiReferenceJava/BidiPBAReference.java
 //
 // The implementation in this file covers definitions BD14-BD16 and rule N0
 // of UAX#9.
@@ -246,7 +246,7 @@ func (p *bracketPairer) getStrongTypeN0(index int) Class {
 // assuming the given embedding direction.
 //
 // It returns ON if no strong type is found. If a single strong type is found,
-// it returns this this type. Otherwise it returns the embedding direction.
+// it returns this type. Otherwise it returns the embedding direction.
 //
 // TODO: use separate type for "strong" directionality.
 func (p *bracketPairer) classifyPairContent(loc bracketPair, dirEmbed Class) Class {