Add submodule pull to CI config

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From https://git.sadmansk.com/sadmansk/blog
+ 54c5589...0425623 master -> origin/master (forced update)
Checking out 04256233 as master...
$ hugo version
Hugo Static Site Generator v0.16 BuildDate: 2016-06-12T08:47:29Z
$ cd themes/cocoa
$ git submodule update --init --recursive
You need to run this command from the toplevel of the working tree.
ERROR: Build failed: exit code 1