Updated PepitaPackage to 1.20

... because the MSBuild based package restore has problems with the subdirectories encoded by PepitaOackage (e.g. "Lib%5Cportable-net4+sl4+wp7+win8+MonoAndroid16+MonoTouch40%5CNullGuard.dll" instead of "Lib/portable-net4%2Bsl4%2Bwp7%2Bwin8%2BMonoAndroid16%2BMonoTouch40/NullGuard.dll"). The current version (I compiled https://github.com/SimonCropp/Pepita/commit/a56a2101128de122ff9097b282487706a800ab44) solves this problem.