Changing dev support ad

Tweaks & renaming developer support ad
Merge pull request #119 from gocardless/change-job-ads

Changing dev support ad
Tweaks to engineering job ads
Removing leavers from about page

Andrew, Dominik & Astrid
Merge pull request #121 from gocardless/removing-leavers

Removing leavers from about page
Fix typo
Merge pull request #120 from gocardless/change-job-ads

Tweaks to engineering job ads
Merge dev