Edit: This doesn't apply to a nuspec file but you can make the changes as described in this answer to always get the latest. The change specified only applies to your own machine. For any code that is yours, you could add your own nuget.config file to make this true.
There isn't a keyword for latest version today. There is however a way to say, use from this version and beyond. You can find the full reference here. Note, this does not mean that NuGet will always get the latest version for the listed dependencies.
An example (from the link provided):
<!-- Accepts any version above, but not include 4.1.3. This might be
used to guarantee a dependency with a specific bug fix. -->
<dependency id="ExamplePackage" version="(4.1.3,)" />
<!-- Accepts any version up below 5.x, which might be used to prevent
pulling in a later version of a dependency that changed its interface.
However, this form is not recommended because it can be difficult to
determine the lowest version. -->
<dependency id="ExamplePackage" version="(,5.0)" />
<!-- Accepts any 1.x or 2.x version, but no 0.x or 3.x and higher versions -->
<dependency id="ExamplePackage" version="[1,3)" />
<!-- Accepts 1.3.2 up to 1.4.x, but not 1.5 and higher. -->
<dependency id="ExamplePackage" version="[1.3.2,1.5)" />