Switch to Go modules

Let's keep the vendor directory for a while to not break Task for
users that install it using `go get`.

Closes #119

go.mod

@@ -0,0 +1,23 @@
+module github.com/go-task/task
+
+require (
+	github.com/Masterminds/semver v0.0.0-20180807142431-c84ddcca87bf
+	github.com/Masterminds/sprig v0.0.0-20180725212158-77bb58b7f5e1
+	github.com/aokoli/goutils v1.0.1 // indirect
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/google/uuid v1.0.0 // indirect
+	github.com/huandu/xstrings v1.1.0 // indirect
+	github.com/imdario/mergo v0.3.6 // indirect
+	github.com/mattn/go-zglob v0.0.0-20180803001819-2ea3427bfa53
+	github.com/mitchellh/go-homedir v1.0.0
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	github.com/radovskyb/watcher v0.0.0-20180525001723-0d9d32686dbf
+	github.com/spf13/pflag v1.0.3
+	github.com/stretchr/testify v1.2.2
+	golang.org/x/crypto v0.0.0-20180830192347-182538f80094 // indirect
+	golang.org/x/net v0.0.0-20180826012351-8a410e7b638d // indirect
+	golang.org/x/sync v0.0.0-20180314180146-1d60e4601c6f
+	golang.org/x/sys v0.0.0-20180831094639-fa5fdf94c789 // indirect
+	gopkg.in/yaml.v2 v2.2.1
+	mvdan.cc/sh v0.0.0-20180829163519-3a244a89e2e5
+)