Add submodule pull to CI config

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HEAD is now at 0425623 Add submodule pull to CI config
From https://git.sadmansk.com/sadmansk/blog
+ 0425623...b45a134 master -> origin/master (forced update)
Checking out b45a1346 as master...
$ hugo version
Hugo Static Site Generator v0.16 BuildDate: 2016-06-12T08:47:29Z
$ git submodule update --init --recursive
No submodule mapping found in .gitmodules for path 'themes/cocoa'
ERROR: Build failed: exit code 1