.travis.yml: make git submodule update conditional.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>

.travis.yml

@@ -3,11 +3,16 @@ sudo: required
 
 language: c
 cache: ccache
+git:
+    submodules: false
 
 before_install:
     - if [ -n "$COVERALLS" ]; then
           pip install --user cpp-coveralls;
       fi;
+    - if expr "$CONFIG_OPTS" ":" ".*enable-external-tests" > /dev/null; then
+          git submodule update --init --recursive;
+      fi;
 
 addons:
     apt: