Make sure the 7z command is available prior to using it

.travis.yml

@@ -8,7 +8,8 @@ branches:
 
 before_install:
   # required libs for android build tools
-  # sudo apt-get update
+  - sudo apt-get update
+  - sudo apt-get install -qq p7zip-full
   # sudo apt-get install -qq --force-yes libgd2-xpm ia32-libs ia32-libs-multiarch
   # newest Android NDK
   - wget http://dl.google.com/android/ndk/android-ndk-r10c-linux-x86_64.bin -O ndk.bin