doc: drop Ubuntu 15.04 (Vivid Vervet)
doc: drop Ubuntu 15.04 (Vivid Vervet)

ref: 11606b5f40b0f56fd801c4c382885100f16c686e

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