End support for NET 4.5 (in favor of NET 4.5.2)

[TimothyMothra]

starting January 12, 2016 Microsoft will no longer provide security updates, technical support or hotfixes for .NET 4, 4.5, and 4.5.1 frameworks.
https://devblogs.microsoft.com/dotnet/support-ending-for-the-net-framework-4-4-5-and-4-5-1/

We should have a serious discussion about migrating our projects off of .NET 4.5 and onto .NET 4.5.2 instead.

ApplicationInsights-dotnet/src/Microsoft.ApplicationInsights/Microsoft.ApplicationInsights.csproj

Line 6 in 9405f13

<TargetFrameworks>net45;net46;netstandard1.3;netstandard2.0</TargetFrameworks>

I expect that dropping support for NET 4.5 would be controversial.
As a compromise, I think we could target both 4.5 and 4.5.2 until we're ready to retire 4.5 completely.

TODO:

1. Add Net452 to all projects targeting net45x
2. Remove Net45 and Net451
3. Audit Builds and confirm that tests are being discovered.

[eddynaka]

@TimothyMothra , i can start adding .net452 where we have .net45, so, when we decide to drop .net45 people that uses at least net452 won't be affected. what do you think?