build: Unpin GMime back to mainline for nightly Flatpak

It's had a pre-release version bump, so we can now tell the difference
if it's been built with the recent incompatible GI/VAPI changes.

See !699

org.gnome.Geary.json

@@ -207,7 +207,7 @@
                 {
                     "type": "git",
                     "url": "https://github.com/jstedfast/gmime.git",
-                    "branch": "3.2.7"
+                    "branch": "master"
                 }
             ]
         },

org.gnome.Geary.yaml

@@ -211,9 +211,7 @@ modules:
     sources:
       - type: git
         url: https://github.com/jstedfast/gmime.git
-        # Pin to 3.2.7 until 3.2.8 is released, or the mainline branch
-        # is pre-bumped to same.
-        branch: 3.2.7
+        branch: master
 
   # Geary dependency
   - name: libpeas