Switch to resetting our SHA along to origin/master

Instead of doing a pull / rebase where we can get out of sync on the
builder instead fetch and reset to the upstream version. This way we
know that we will reset back to a known state rather than relying on
previous pulls and rebase to go forwards

buildconfig/Jenkins/merge-release-next.sh

 #!/bin/sh
+git fetch origin --prune
 git checkout master
-git pull --rebase
+git reset --hard origin/master
 
 git config user.name mantid-builder
 git config user.email "mantid-buildserver@mantidproject.org"