Fix broken CI Builds

[BillyONeal]

Address a smorgasbord of issues caused by Azure Pipelines updating their hosted VMs out from underneath us:

• Azure pipelines deployed boost 1.69 into their VMs, but the version of GCC they have in that release hates that version of Boost. We unset BOOST_ROOT to use the default 1.58 version. We always want the version of boost in apt for the version of Ubuntu you get on Azure boxes to work with cpprestsdk, so we didn't want to jump through hoops to take the new 1.69 dependency here.
• Updated vcpkg and boost-for-android which fixed MacOS and Android builds, respectively due to MacOS and Android NDK updates.
• Disabled iOS for now because the Apple-Boost-BuildScript we use there appears to be unmaintained and broken on the XCode used in Pipelines; we would love to see contributors ( @mobileben ? ) who know what's going on flip this back on :)

[BillyONeal]

@mobileben Do you know what's making iOS unhappy? https://dev.azure.com/vclibs/cpprestsdk/_build/results?buildId=412