Because we might use dependencies soon that require this framework.
And anyway these days the .NET world is mostly a hybrid between
.NETStandard/.NETCore libs and .NET4.6 ones, not older than that.
This change is good because:
a) It's better to target a lower target framework version if
the project doesn't necessarily depend on the new features
of the newer versions (so, bigger target audience).
b) It lets compile the project with implementations of the
.NET Framework that are not compatible with 4.5.2. For example,
after this change I can successfully build TLSharp with the
Mono v4.2.1 that comes in my Ubuntu Linux 16.04.1.
