doc: update sphinx theme name
doc: update sphinx theme name

Since 1.3b1 release, theme name is changed to 'classic'.

See http://sphinx-doc.org/changes.html#release-1-3b3-released-feb-24-2015

yoku0825/mroonga,mroonga/mroonga,yoku0825/mroonga,naoa/mroonga,naoa/mroonga,naoa/mroonga,yoku0825/mroonga,naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga,mroonga/mroonga,naoa/mroonga,mroonga/mroonga,yoku0825/mroonga,naoa/mroonga,mroonga/mroonga