Travis-CI: enable ccache for faster builds

[rouault]

Cold run (cache empty) with gcc: 8 min 55 s
Hot run (cache filled) with gcc: 5 min 58s

Cold run (cache empty) with clang: 9 min 55 sec
Hot run (cache filled) with clang: 6 min 44 sec

[wenzeslaus]

Thanks, this is great!

After looking into the documentation, isn't cache: ccache enough?

https://docs.travis-ci.com/user/caching/#ccache-cache

[rouault]

isn't cache: ccache enough?

just changed to that. I followed GDAL .travis.yml example, but Travis syntax may have improved in the meantime.