Target master with release PR

`master` is now targetted by the release PR instead of `develop`, as
the release has to be created from the master branch.

The update to `develop` is handled after the release by a PR from
`master` to `develop`, which is created automatically after the
release.

.circleci/config.yml

@@ -240,6 +240,9 @@ jobs:
           name: Create GitHub release
           command: |
             .circleci/scripts/release-create-gh-release
+      - run:
+          name: Create GitHub Pull Request to sync master with develop
+          command: .circleci/scripts/release-create-master-pr
       # - run:
       #    name: github gh-pages docs publish
       #    command: >

.circleci/scripts/release-create-master-pr

@@ -0,0 +1,53 @@
+#!/usr/bin/env bash
+
+set -e
+set -u
+set -o pipefail
+
+if [[ "${CI:-}" != 'true' ]]
+then
+    printf '%s\n' 'CI environment variable must be set to true'
+    exit 1
+fi
+
+if [[ "${CIRCLECI:-}" != 'true' ]]
+then
+    printf '%s\n' 'CIRCLECI environment variable must be set to true'
+    exit 1
+fi
+
+if [[ -z "${GITHUB_TOKEN:-}" ]]
+then
+    printf '%s\n' 'GITHUB_TOKEN environment variable must be set'
+    exit 1
+fi
+
+function install_github_cli ()
+{
+    printf '%s\n' 'Installing hub CLI'
+    pushd "$(mktemp -d)"
+        curl -sSL 'https://github.com/github/hub/releases/download/v2.11.2/hub-linux-amd64-2.11.2.tgz' | tar xz
+        PATH="$PATH:$PWD/hub-linux-amd64-2.11.2/bin"
+    popd
+}
+
+base_branch='develop'
+
+if [[ -n "${CI_PULL_REQUEST:-}" ]]
+then
+    printf '%s\n' 'CI_PULL_REQUEST is set, pull request already exists for this build'
+    exit 0
+fi
+
+install_github_cli
+
+printf '%s\n' "Creating a Pull Request to sync 'master' with 'develop'"
+
+if ! hub pull-request \
+    --reviewer '@MetaMask/extension-release-team' \
+    --message "Master => develop" --message 'Merge latest release back into develop' \
+    --base "$CIRCLE_PROJECT_USERNAME:$base_branch" \
+    --head "$CIRCLE_PROJECT_USERNAME:$CIRCLE_BRANCH";
+then
+    printf '%s\n' 'Pull Request already exists'
+fi

.circleci/scripts/release-create-release-pr

@@ -32,7 +32,7 @@ function install_github_cli ()
 }
 
 version="${CIRCLE_BRANCH/Version-v/}"
-base_branch='develop'
+base_branch='master'
 
 if [[ -n "${CI_PULL_REQUEST:-}" ]]
 then