Resurrect requirements_vagrant.txt as requirements_migrations.txt

The file was removed in af3c381d to remove Vagrant, but it served a
dual purpose as requirements to test our suite of migrations. It's
resurrection will hopefully let Travis pass again.

Makefile

@@ -76,7 +76,7 @@ lint:
 	./lint.sh
 
 testmigrations:
-	pip install -r requirements_vagrant.txt
+	pip install -r requirements_migrations.txt
 	cd sites/sandbox && ./test_migrations.sh
 
 # This target is run on Travis.ci. We lint, test and build the sandbox/demo sites as well 

requirements_migrations.txt

@@ -0,0 +1,3 @@
+# Those dependencies are needed for the test_migrations.sh script
+PyMySQL==0.6.2
+psycopg2==2.4.5