TRAVIS: enable ccache compression

Current builds saturate Travis CI caches:

```
$ diff -U999 /tmp/ccache_before /tmp/ccache_after || true
--- /tmp/ccache_before	2019-11-19 21:45:52.491212479 +0000
+++ /tmp/ccache_after	2019-11-19 21:58:32.335898083 +0000
@@ -1,10 +1,13 @@
 cache directory                     /home/travis/.ccache
 primary config                      /home/travis/.ccache/ccache.conf
 secondary config      (readonly)    /etc/ccache.conf
 cache hit (direct)                     0
 cache hit (preprocessed)               0
-cache miss                             0
-no input file                          1
-files in cache                         0
-cache size                           0.0 kB
+cache miss                          4156
+called for link                       66
+called for preprocessing               3
+compile failed                         9
+no input file                          2
+files in cache                     10568
+cache size                         434.1 MB
 max cache size                     500.0 MB
```

I doubt it's possible to get more space allocated, though there are no
docs on what exact limits are. I suspect that default explicit setting
matches what's provided. Instead of trying to get more storage,
compress the objects with zlib (default level 6) to try increasing hit
rate.

.travis.yml

@@ -78,6 +78,7 @@ dist: xenial
 script:
   - ccache --show-stats > /tmp/ccache_before
   - export PATH="/usr/local/opt/ccache/libexec:/usr/lib/ccache:$PATH"
+  - export CCACHE_COMPRESS=1
   - ./configure --enable-all-engines --enable-opl2lpt
   - make -j 2
   - make test