Add version scoped url to go.mod and install instructions

[bhelx]

golang/go#35732

It looks like our install instructions fall into this pseudo-version
trap mentioned in the above issue. This adds the v3 scope to the url
and it updates the bump script to make sure that the number stays up to
date.

Testing

testing that the bump script is updating the right stuff. Should only change things on major version bumps:

[I] ⋊> ~/r/v/c/go  ./scripts/bump major                                                                                                                                          
[I] ⋊> ~/r/v/c/go ⨯ git diff                                                                                                                                                     

diff --git a/.bumpversion.cfg b/.bumpversion.cfg
index 1877afb..9b66879 100644
--- a/.bumpversion.cfg
+++ b/.bumpversion.cfg
@@ -1,5 +1,5 @@
 [bumpversion]
-current_version = 3.0.0
+current_version = 4.0.0
 parse = (?P<major>\d+)
        \.(?P<minor>\d+)
        \.(?P<patch>\d+)
diff --git a/README.md b/README.md
index 784c055..5edcfdd 100644
--- a/README.md
+++ b/README.md
@@ -11,14 +11,14 @@ Documentation for the HTTP API and example code can be found [on our Developer H
 Install using `go get`:

 \`\`\`
-go get -u github.com/recurly/recurly-client-go/v3
+go get -u github.com/recurly/recurly-client-go/v4
 \`\`\`

 import with:

 \`\`\`go
 import (
-    "github.com/recurly/recurly-client-go/v3"
+    "github.com/recurly/recurly-client-go/v4"
 )
 \`\`\`

diff --git a/go.mod b/go.mod
index 69a2af1..bc8e71b 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,3 @@
-module github.com/recurly/recurly-client-go/v3
+module github.com/recurly/recurly-client-go/v4

 go 1.12
diff --git a/version.go b/version.go
index 625e5ec..d37aeba 100644
--- a/version.go
+++ b/version.go
@@ -1,5 +1,5 @@
 package recurly

 const (
-       clientVersion = "3.0.0"
+       clientVersion = "4.0.0"
 )