build: Update CI infrastructure for "master" -> "main" rename

Also remove obsolete Travis build status from the README.md.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>

.github/workflows/build.yaml

@@ -31,7 +31,7 @@ jobs:
 
       - name: Debug Check (default features)
         run: |
-          git rev-list origin/master..$GITHUB_SHA | xargs -t -I % sh -c 'git checkout %; cargo check --all --target=${{ matrix.target }}'
+          git rev-list origin/main..$GITHUB_SHA | xargs -t -I % sh -c 'git checkout %; cargo check --all --target=${{ matrix.target }}'
           git checkout $GITHUB_SHA
 
       - name: Build (acpi,kvm)

.github/workflows/docker-image.yaml

@@ -2,7 +2,7 @@ name: Cloud-Hypervisor's Docker image update
 
 on:
   push:
-    branches: master
+    branches: main
     paths: resources/Dockerfile
 
 jobs: