Have Travis-CI fetch submodules recursively

By default it just does `git submodule init`

.travis.yml

@@ -59,6 +59,7 @@ install:
   - git clone git://github.com/llvm-mirror/clang.git ~/pyston_deps/llvm-trunk/tools/clang
   - git config --global user.email "you@example.com"
   - git config --global user.name "Your Name"
+  - git submodule update --init --recursive
   - mkdir ~/pyston-build && cd ~/pyston-build
   - make -C $TRAVIS_BUILD_DIR llvm_up
   - bash $TRAVIS_BUILD_DIR/tools/configure_travis.sh