EDIT 2018: Only follow the instructions for updating the package if you really know what you are doing. In most cases, you never need to update this package – or other packages marked as “blocked by project” – manually. A framework-dependent app will use the latest runtime available and a self-contained application will perform an extra build using a newer version of this package automatically. (there are some edge cases where you need to upgrade this package in test projects. in this case, add <TargetLatestRuntimePatch>true</…> and see this Q&A for other options)
The implicit package references that the Microsoft.NET.Sdk infers can’t be updated via NuGet.
If you migrated from project.json, the project with the 1.1.0 reference likely contains
<RuntimeFrameworkVersion>1.1.0</RuntimeFrameworkVersion>
in the csproj file or an item like this (if you may used the package manager previously to set the version):
<PackageReference Update="Microsoft.NETCore.App" Version="1.1.0" />
Delete entries like the above and all packages will reference 1.1.2 (or whatever the installed SDK considers to be the latest) automatically.
Alernatively, set RuntimeFrameworkVersion in all projects.