Switch from govendor to go modules. (#587)

Thanks to @BenoitKnecht for leading the way on this.

vendor/github.com/urfave/cli/.flake8

@@ -0,0 +1,2 @@
+[flake8]
+max-line-length = 120

vendor/github.com/urfave/cli/.gitignore

@@ -0,0 +1,2 @@
+*.coverprofile
+node_modules/

vendor/github.com/urfave/cli/.travis.yml

@@ -0,0 +1,27 @@
+language: go
+sudo: false
+dist: trusty
+osx_image: xcode8.3
+go: 1.8.x
+
+os:
+- linux
+- osx
+
+cache:
+  directories:
+  - node_modules
+
+before_script:
+- go get github.com/urfave/gfmrun/... || true
+- go get golang.org/x/tools/cmd/goimports
+- if [ ! -f node_modules/.bin/markdown-toc ] ; then
+    npm install markdown-toc ;
+  fi
+
+script:
+- ./runtests gen
+- ./runtests vet
+- ./runtests test
+- ./runtests gfmrun
+- ./runtests toc