chore(contributing): Update which branch to submit PRs against.

Author: deeg

CONTRIBUTING.md

@@ -28,7 +28,7 @@ Unfortunately we are not able to investigate / fix bugs without a minimal reprod
 
 We are always looking for the quality contributions and will be happy to accept your Pull Requests as long as those adhere to some basic rules:
 
-* Please open all pull requests against the `dev` branch.
+* Please open all pull requests against the `master` branch.
 * Please assure that you are submitting quality code, specifically make sure that:
   * You have accompanying tests and all the tests are passing.  See testing below.
   * Your PR doesn't break the build; check the Travis-CI build status after opening a PR and push corrective commits if anything goes wrong