Pin monad and pester submodules to develop branchs

This only affects updating with `git submodule update --remote`, which
will fetch and checkout the corresponding branch (defaulting to master
for most of our submodules) instead of the recorded commit.

Note that the submodule will still be checked out to a detached head, as
it should be.

.gitmodules

@@ -1,6 +1,7 @@
 [submodule "src/monad"]
 	path = src/monad
 	url = https://msostc.visualstudio.com/DefaultCollection/PS/_git/monad
+	branch = develop
 [submodule "ext-src/cppunit"]
 	path = ext-src/cppunit
 	url = git://anongit.freedesktop.org/git/libreoffice/cppunit/
@@ -13,6 +14,7 @@
 [submodule "ext-src/pester"]
 	path = ext-src/pester
 	url = https://msostc.visualstudio.com/DefaultCollection/PS/_git/Pester
+	branch = develop
 [submodule "src/monad-omi-provider"]
 	path = src/monad-omi-provider
 	url = https://msostc.visualstudio.com/DefaultCollection/PS/_git/monad-omi-provider