refactor: default versionScheme to ‘semver’

lib/manager/bazel/extract.js

lib/manager/buildkite/extract.js

lib/manager/meteor/extract.js

lib/manager/npm/extract/index.js

lib/manager/nvm/extract.js

lib/manager/pip_requirements/extract.js

lib/workers/repository/updates/flatten.js

test/manager/bazel/__snapshots__/extract.spec.js.snap

test/manager/buildkite/__snapshots__/extract.spec.js.snap

test/manager/meteor/__snapshots__/extract.spec.js.snap

test/manager/npm/extract/__snapshots__/index.spec.js.snap

test/manager/nvm/__snapshots__/extract.spec.js.snap

...nager/pip_requirements/__snapshots__/extract.spec.js.snap