Make VersionTo, VersionFrom and PreRelease part of package material identity

Fixes https://github.com/ThoughtWorksInc/go-nuget-poller-plugin/issues/9
Added missing tests for INCLUDE_PRE_RELEASE field
Updating hardcoded version in test.

Needs solving in a better way, as this will frequently fail as new versions of 1Password are released
Fixing NullPointerException when no new packages found during modificationSince call
Updating version in plugin xml