Might happen if CacheManager.Core is only implicitly referenced. For reasons, nuget installs the beta package of 1.0.0.

Workaround
To work around this until it is fixed in 1.0.1, add a NuGet dependency to CacheManager.Core version 1.0.0. To either csproj or project.json.
<PackageReference Include="CacheManager.Core" Version="1.0.0" />