BUILD: Speed up Travis-CI builds

container-based infrastructure (by sudo: false) starts and runs
faster:
https://docs.travis-ci.com/user/migrating-from-legacy/

gcc and make are preinstalled by Travis-CI for C++ language
containers, so do not need to be listed as dependencies:
https://docs.travis-ci.com/user/trusty-ci-environment/#Compilers-%26-Build-toolchain

ccache caching prevents wasting time rebuilding unchanged code:
https://docs.travis-ci.com/user/caching#ccache-cache

.travis.yml

@@ -1,12 +1,12 @@
 language:
   - cpp
 
-sudo: required
+sudo: false
+cache: ccache
 
 addons:
   apt:
     packages:
-    - g++ make
     - libsdl2-dev
     - libsdl2-net-dev
     - libcurl4-openssl-dev