all: convert to a Go module

And remove vendor/, as it's now unnecessary. go.sum ensures that
dependencies aren't tampered with, and proxy.golang.org keeps copies of
all the archives.

.travis.yml

@@ -6,11 +6,12 @@ addons:
 
 language: go
 
-go_import_path: gopkg.in/Netflix-Skunkworks/go-jira.v1
-
 go:
   - 1.12.x
 
+env:
+  - GO111MODULE=on
+
 script:
   - go test ./...
   - go vet -composites=false ./...