.gitmodules: Ignore changed submodules

When committing changes, ignore submodules that
have received changes.

.gitmodules

@@ -2,6 +2,8 @@
 	path = source/images-translated
 	url = https://git.mahara.org/user-manual/manual-images.git
 	branch = 17.04_STABLE
+	ignore = dirty
 [submodule "manual-build"]
 	path = manual-build
 	url = https://git.mahara.org/user-manual/manual-build.git
+	ignore = dirty