travis: do not error on making cache folder, if it already restored from cache

.travis.yml

@@ -23,7 +23,7 @@ addons:
     - p7zip-full
 
 before_script:
-  - mkdir $HOME/srb2_cache
+  - mkdir -p $HOME/srb2_cache
   - wget --verbose --server-response -c http://rosenthalcastle.org/srb2/SRB2-v2114-assets.7z -O $HOME/srb2_cache/SRB2-v2114-assets.7z
   - 7z x $HOME/srb2_cache/SRB2-v2114-assets.7z -oassets
   - mkdir build