ci(release): Add .version file (#170)

### Changes

This PR:

- Adds a `.version` file as part of our updated release process.
- Updates `.shiprc` to ensure the new file is bumped during releases.
- Updates `.shiprc` to properly reflect versions are bumped in
`src/Service.php` now. (Ship has not traditionally been used to cut
releases for this SDK, so I am bringing this up to date now as I
intended to use that moving forward.)

### References

N/A

### Testing

N/A

### Contributor Checklist

- [x] I agree to adhere to the [Auth0 General Contribution
Guidelines](https://github.com/auth0/open-source-template/blob/master/GENERAL-CONTRIBUTING.md).
- [x] I agree to uphold the [Auth0 Code of
Conduct](https://github.com/auth0/open-source-template/blob/master/CODE-OF-CONDUCT.md).

.shiprc

@@ -1,6 +1,7 @@
 {
   "files": {
-    "src/Auth0Bundle.php": []
+    "src/Service.php": [],
+    ".version": []
   },
   "prefixVersion": false
 }

.version

@@ -0,0 +1 @@
+5.1.0