Remove Mono 3.2.8 from Mono testing

[dbent]

@pjf @techman83
#1645 is failing in Travis on the Mono 3.2.8 version because of errors like these:

System.TypeInitializationException : An exception was thrown by the type initializer for CKAN.Versioning.KspVersionRange
  ----> System.TypeInitializationException : An exception was thrown by the type initializer for CKAN.Versioning.KspVersionBound
  ----> System.NullReferenceException : Object reference not set to an instance of an object

There is no reason why a NullReferenceException should occur in the type initializer for KspVersionBound, this suggests a bug in the Mono 3.2.8 runtime:

• The error does not occur on Mono 3.10.0, Mono 4.2.3, or the .NET runtime.
• A similar error was fixed in Mono in the 3.2.x era.

Do we even support running CKAN on such an old version of Mono?

[politas]

No. Mono 4.0 / .Net 4.5 is the minimum now.

[techman83]

I think the intention was for the core to be compliant as far back as practical. We are leaning on the newer APIs in the core already from what I recall. It's a small change to turn it back on if required.