Convert build system to go modules. (google#544)

* go: Switch to go version 1.13.

1.11 is unsupported, and 1.12 doesn't default to modules which makes
testing more complicated.

* Replace Gopkg with go modules

* Go mod tidy

* go get -u=patch ./...

* go: Add go module directory to travis cache to speed up tests

* Modify update-deps to generate vendor directory and check it in.

This should allow go1.12 (which will default to no modules) to work.

Author: clrprod

.travis.yml

@@ -6,12 +6,16 @@ language:
   - go
 
 go:
-  - "1.11"
   - "1.12"
+  - "1.13"
 
 git:
   depth: 1
 
+cache:
+  directories:
+    - $GOPATH/pkg/mod
+
 install:
   # Lint!
   - go get -v -u golang.org/x/lint/golint