Clear changes to schema.rb when upgrading

Having local changes is quite common and it is a stumbling block when
upgrading GitLab. Because schema.rb is generated from the actual DB
schema it is OK to clear the local changes.
Merge branch 'checkout_schema.rb' into 'master'

Clear changes to schema.rb when upgrading

See merge request !1142
Merge branch 'master' of github.com:gitlabhq/gitlabhq