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
Change revision string to be the package version.
Fix tests