Merge branch 'add-ci_variables-environment_scope' into 'master'

app/models/ci/variable.rb

...s/unreleased/add-ci_variables-environment_scope-mysql.yml

db/migrate/20170622135451_rename_duplicated_variable_key.rb

...e/20170622135628_add_environment_scope_to_ci_variables.rb

...e/20170622135728_add_unique_constraint_to_ci_variables.rb

...te/20170623080805_remove_ci_variables_project_id_index.rb

db/schema.rb

spec/migrations/rename_duplicated_variable_key_spec.rb

spec/models/ci/variable_spec.rb