Fix spirit based mana regen

Fix spirit based mana regen because mana regen was too quick, especially
with spells like innervate or evocate or when players have a high ratio
spirit/mana points.
Unlike TBC and later expansion, spirit mana regen values per class were
not stored in DBC in Classic but used formulas as described below
http://www.wowwiki.com/Mana_regeneration?direction=prev&oldid=103895
However, those formulas give regen values per tick, a tick occuring
every 2 sec, the returned value must be cut by half.
source: http://www.wowwiki.com/Talk:Mana_regeneration#Untitled