Disable deprecation for MSVC 2017

Although technically supported, this version of the visual studio likes to put the warnings where templates are defined (rather than instantiated). That makes it impossible to suppress a warning when, for example, when a deprecated class is legitmately used in an std template to implement other deprecated functionality.

To prevent this, disable deprecations on MSVC compilers before 2019. I doubt any developers are using this compiler anywhere but the dashboard, and the warnings should legitimately appear elsewhere.