CONTRIBUTING.md: Remove the bit about the testing branch

We're going to commit directly to master from now one.

The 'testing' policy predates PyPI releases and Git master was the primary mean
of distributing Limnoria back then, but it does not make sense anymore.

Author: progval

CONTRIBUTING.md

@@ -19,10 +19,6 @@ optional.
 
 ## Sending patches
 
-When you send a pull request, **send it to the testing branch**. 
-It will be merged to master when it's considered to be stable enough to be 
-supported.
-
 Don't fear that you spam Limnoria by sending many pull requests. According 
 to @ProgVal, it's easier for them to accept pull requests than to 
 cherry-pick everything manually.