Remove dependency ranges from NuGet package specification

Fixes #433

Currently the NuGet package specification is written with dependency ranges of the form `[5.0.8,6.0)`. While this range is the correct range for users that are not using assembly binding redirection, including the upper limit fully prevents users from installing higher versions and using assembly binding redirection to resolve conflicts.

By changing the version back to the form `5.0.8` (which is equivalent to `[5.0.8,)`), users of the library are responsible for choosing the correct version of the underlying dependency, and for including assembly binding redirection. This change fixes #433 without breaking existing code for users referencing Json.NET 5.x.

Note that **SimpleRESTServices** includes a transitive dependency on Json.NET 5.x, but no upper bound is specified in that dependency so no changes were required in that library to implement this functionality.