Fixes error when '~/.vault-token' contains a trailing newline

First of all, thanks for the great tool!

This PR fixes an error encountered by one of our developers which occured when they had manually created the `~/.vault-token` file, and their editor had added a trailing newline. The error comes from the following line: https://github.com/hashicorp/vault/blob/master/api/client.go#L747

The official vault client was patched for this in hashicorp/vault@f8c657a, after issues hashicorp/vault#1774 and hashicorp/vault#1902.

cmd/root.go

@@ -116,5 +116,5 @@ func vaultTokenFromDisk() string {
 		return ""
 	}
 
-	return string(data)
+	return strings.TrimSpace(string(data))
 }