Ubuntu 14.04 warnings & !compile

Ubuntu 14.04 if(!TellAFriend.Enabled)return;  cause these Unreachables codes

+ Custom Systems/Referral Rewards/ReferralRewardItems.cs:
    CS0162: Line 86: Unreachable code detected
 + Custom Systems/Referral Rewards/ReferralRewardsStone.cs:
    CS0162: Line 36: Unreachable code detected
 + Custom Systems/Referral Rewards/TellAFriend.cs:
    CS0162: Line 31: Unreachable code detected
Merge pull request #4 from frkdom/master

Ubuntu 14.04 warnings & !compile