minor #294 Update supported branches in README file and PR template (GSadee)

This PR was merged into the 1.5 branch.

Discussion
----------

| Q               | A
| --------------- | -----
| Branch?         | 1.5
| Bug fix?        | no
| New feature?    | no
| Related tickets | 


Commits
-------
  Update supported branches in README file
  Update PR template

Author: NoResponseMate

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,6 @@
 | Q               | A
 | --------------- | -----
-| Branch?         | 1.3 (security fixes), 1.4 or 1.5 (bug fixes, improvements), master (new features)
+| Branch?         | 1.5 (security fixes), 1.6 (bug fixes, improvements), 1.7 (new features)
 | Bug fix?        | no/yes
 | New feature?    | no/yes
 | Related tickets | fixes #X, partially #Y, mentioned in #Z

README.md

@@ -51,7 +51,6 @@ Sylius Core Team’s plugin for [PayPal Commerce Platform](https://www.paypal.co
 
 #### Supported branches
 
-* `1.3` (v1.3.*) - security fixes
-* `1.4` (v1.4.*) - bug fixes
-* `1.5` (v1.5.*) - bug fixes, improvements
-* `master` (v1.6.*) - new features
+* `1.5` (v1.5.*) - security fixes
+* `1.6` (v1.6.*) - bug fixes, improvements
+* `1.7` (v1.7.*) - new features