Remove references to `master` from CI

- [x] remove master from github actions
- [x] remove master from .travis.yml
- [x] make `develop` the default branch for `spack ci`

.github/workflows/linux_build_tests.yaml

@@ -3,13 +3,12 @@ name: linux builds
 on:
   push:
     branches:
-    - master
     - develop
     - releases/**
   pull_request:
     branches:
-    - master
     - develop
+    - releases/**
     paths-ignore:
       # Don't run if we only modified packages in the built-in repository
       - 'var/spack/repos/builtin/**'

.github/workflows/macos_unit_tests.yaml

@@ -3,13 +3,12 @@ name: macos tests
 on:
   push:
     branches:
-      - master
       - develop
       - releases/**
   pull_request:
     branches:
-      - master
       - develop
+      - releases/**
 jobs:
   build:
 

.github/workflows/minimum_python_versions.yaml

@@ -3,13 +3,12 @@ name: python version check
 on:
   push:
     branches:
-      - master
       - develop
       - releases/**
   pull_request:
     branches:
-      - master
       - develop
+      - releases/**
 jobs:
   validate:
 

.travis.yml

-# Only build master and develop on push; do not build every branch.
+# Only build releases and develop on push; do not build every branch.
 branches:
   only:
-    - master
     - develop
     - /^releases\/.*$/
 

lib/spack/spack/ci.py

@@ -493,7 +493,7 @@ def generate_gitlab_ci_yaml(env, print_summary, output_file,
     after_script = None
     if custom_spack_repo:
         if not custom_spack_ref:
-            custom_spack_ref = 'master'
+            custom_spack_ref = 'develop'
         before_script = [
             ('git clone "{0}"'.format(custom_spack_repo)),
             'pushd ./spack && git checkout "{0}" && popd'.format(