Updated to OpenTK 1.1.3

Updated OpenTK to version 1.1.3, released 20 June 2014. This is a bugfix release.

Fixes issues https://github.com/mono/MonoGame/issues/2686 and https://github.com/mono/MonoGame/issues/2711

May affect issue https://github.com/mono/MonoGame/issues/2664