CI: use branch, not refspec

Kas 3.3 uses branch and commit keys instead of a general-purpose
refspec. Change the base to use branch:mickledore and remove the
explicit use of master for meta-virtualization now that a mickledore
branch exists there too now.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

ci/base.yml

@@ -5,7 +5,7 @@ distro: poky
 
 defaults:
   repos:
-    refspec: mickledore
+    branch: mickledore
 
 repos:
   meta-arm:

ci/meta-virtualization.yml

@@ -6,4 +6,3 @@ header:
 repos:
   meta-virtualization:
     url: git://git.yoctoproject.org/meta-virtualization
-    refspec: master