Use framework constraint for paket.dependencies in convert-from-nuget

[mausch]

Now that #574 works (thanks!) it would be great if Paket could insert framework constraints in paket.dependencies where appropriate when calling convert-from-nuget.

Otherwise, a new user would still get the same error after convert-from-nuget until they figure out that they need to insert those constraints.

[forki]

@theimowski what do you think?

[theimowski]

can we read that from the packages.config? If so then that shouldn't be a big deal