travis-ci: guaranteed to fetch tags, by setting depth to 0xffffff

Now with default depth 50, there is no tags, and the next error
happened:
build/config.h:5:34: error: unable to find numeric literal operator ‘operator"" fa6f8.43fa6f8’
 #define BOOSTCACHE_VERSION_FLOAT 43fa6f8.43fa

CI: https://travis-ci.org/azat/boostcache/builds/27408644