Rename master branch to main

In response to the endorsed proposal: [TGG: Address offensive and objectionable technology terms](https://docs.google.com/document/d/1v6z7_NkLFeYAFotTYB8C1OguO5iMHHLpAOv_yG5EKHI
).

Once this is merged I'll do the rest of the steps documented here:
https://github.com/Financial-Times/next/wiki/Migrating-apps-to-use-main-branch-(instead-of-master)

.circleci/config.yml

@@ -31,13 +31,13 @@ references:
     restore_cache:
         <<: *npm_cache_keys
 
-  filters_only_master: &filters_only_master
+  filters_only_main: &filters_only_main
     branches:
-      only: master
+      only: main
 
-  filters_ignore_master: &filters_ignore_master
+  filters_ignore_main: &filters_ignore_main
     branches:
-      ignore: master
+      ignore: main
 
   filters_ignore_tags: &filters_ignore_tags
     tags:
@@ -146,7 +146,7 @@ workflows:
       - schedule:
           cron: "0 0 * * *"
           filters:
-            <<: *filters_only_master
+            <<: *filters_only_main
     jobs:
       - build:
           context: next-nightly-build

README.md

@@ -1,6 +1,6 @@
 # n-es-client
 
-[![CircleCI](https://img.shields.io/circleci/project/github/Financial-Times/n-es-client/master.svg)](https://circleci.com/gh/Financial-Times/n-es-client) ![Coveralls Coverage](https://img.shields.io/coveralls/github/Financial-Times/n-es-client/master.svg) [![NPM version](https://img.shields.io/npm/v/@financial-times/n-es-client.svg)](https://www.npmjs.com/package/@financial-times/n-es-client)
+[![CircleCI](https://img.shields.io/circleci/project/github/Financial-Times/n-es-client/main.svg)](https://circleci.com/gh/Financial-Times/n-es-client) ![Coveralls Coverage](https://img.shields.io/coveralls/github/Financial-Times/n-es-client/main.svg) [![NPM version](https://img.shields.io/npm/v/@financial-times/n-es-client.svg)](https://www.npmjs.com/package/@financial-times/n-es-client)
 
 A very thin wrapper around [signed fetch][1] and [http-errors][2] to search and retrieve content from our Elasticsearch clusters in a simple, DRY manner.