Don't prefix the commit message with branch number. (GH-39)

Only add the branch prefix to the PR title.

Use cherry_picker v0.2.4

Closes python/miss-islington#38

Author: Mariatta

backport/tasks.py

@@ -46,7 +46,8 @@ def backport_task(commit_hash, branch, *, issue_number, created_by, merged_by):
                                cherry_picker {commit_hash} {branch}
                                ```
                                """)
-    cp = cherry_picker.CherryPicker('origin', commit_hash, [branch])
+    cp = cherry_picker.CherryPicker('origin', commit_hash, [branch],
+                                    prefix_commit=False)
     try:
         cp.backport()
     except cherry_picker.BranchCheckoutException:

requirements.txt

@@ -1,4 +1,4 @@
-cherry_picker
+cherry_picker==0.2.4
 aiohttp
 gidgethub
 cachetools