Fix for building against latest develop on maya 2018+

[pmolodo]

Description (this won't be part of the changelog)

The latest develop did not build for me, on any version of maya >= 2018. This was because of a conditional include of a boost header, so that it was only added if maya < 2018.

Changelog

Fixed

• Fixes compilation against maya 2018+

Checklist (Please do not remove this line)

• [X] Make sure the Title and Description of the PR make sense and provide sufficient context for your work
• [X] Do any added files have the correct AL Apache Licence Header?
• [X] Are there Doxygen comments in the headers?
• [X] Are any new features, behaviour changes documented in the .md format documentation?
• [ ] Have you added, updated tests to cover new features and behaviour changes?
• [X] Have you filled out at least one changelog entry?
• [X] Is the branch's history clean? (only relevant commits)

[murphyeoin]

Thanks @elrond79. We run our tests on Maya-2019 internally also. Must be some difference wrt our open source testing environment which is currently maya-2017 only (yes we plan to update whenever we can find a small amount of time)