Stop scripts from being output unnecessarily

I don't use widgets on my site at all but the ajax.js still gets output (and so loaded). Moving the enqueue to the widget output means the file gets output in the footer (which is better for speed) and stops it from being loaded when it's not needed.
Change the version to 3.1.8

Change the version to 3.1.8 - Stop scripts from being output
unnecessarily when we load the widget on the front-end.
BugIssue #1917 - Get the correct users version

BugIssue #1917 - Get the correct users version
Bug issue #1550 - Bug fix related to the compatibility of the plugin for v1 and v3 users

Bug issue #1550 - Bug fix related to the compatibility of the plugin for
v1 and v3 users
Merge pull request #20 from mailjet/development

Merge development with master