Switch from govendor to go modules. (#587)

Thanks to @BenoitKnecht for leading the way on this.

vendor/github.com/vultr/govultr/.codecov.yml

@@ -0,0 +1,8 @@
+coverage:
+  status:
+    project:
+      default:
+        target: 70%
+    patch:
+      default:
+        target: 60%

vendor/github.com/vultr/govultr/.gitignore

@@ -0,0 +1,19 @@
+# Binaries for programs and plugins
+*.exe
+*.exe~
+*.dll
+*.so
+*.dylib
+
+# Test binary, built with `go test -c`
+*.test
+
+# Output of the go coverage tool, specifically when used with LiteIDE
+*.out
+
+### Go Patch ###
+/vendor/
+/Godeps/
+.DS_Store
+.idea
+.vscode

vendor/github.com/vultr/govultr/.travis.yml

@@ -0,0 +1,16 @@
+language: go
+
+go:
+  - 1.12.x
+  - 1.13.x
+  - tip
+
+matrix:
+  allow_failures:
+    - go: tip
+
+script:
+  - go test -race -coverprofile=coverage.txt -covermode=atomic
+
+after_success:
+  - bash <(curl -s https://codecov.io/bash)