Migrate master->main

Change references in CI and docs from 'master' to 'main' as the primary
branch has been renamed.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/ci/imgtool_run.sh b/ci/imgtool_run.sh
index 7358706..97f0b21 100755
--- a/ci/imgtool_run.sh
+++ b/ci/imgtool_run.sh
@@ -13,8 +13,8 @@
 # limitations under the License.
 
 if [[ $TRAVIS == "true" ]]; then
-    if [[ $TRAVIS_PULL_REQUEST != "false" || $TRAVIS_BRANCH != "master" ]]; then
-        echo "Either a PR or not \"master\" branch, exiting"
+    if [[ $TRAVIS_PULL_REQUEST != "false" || $TRAVIS_BRANCH != "main" ]]; then
+        echo "Either a PR or not \"main\" branch, exiting"
         exit 0
     fi
 fi