ci: Travis: ccache: use CCACHE_HASHDIR [skip appveyor]

This is the default since ccache 3.3, but Travis has 3.2.4.

This fixes compiler warnings from macros with later clang/gcc.

Using CCACHE_HASHDIR=1 fixes ccache v3.2.4, but CCACHE_NOHASHDIR=1 does
not break v3.7.1. The real issue/fix appears to be
ccache/ccache@284e3a0, and using the hashdir option seems to only work
around this.

Unblocks neovim#10480, and
neovim#10487.

Ref: ccache/ccache@6d9cb3dfdd9
Closes neovim#10533.

.travis.yml

@@ -46,6 +46,8 @@ env:
     - CCACHE_COMPRESS=1
     - CCACHE_SLOPPINESS=time_macros,file_macro
     - CCACHE_BASEDIR="$TRAVIS_BUILD_DIR"
+    # Default since 3.3, but Travis (Xenial) has 3.2.4; workaround (#10533).
+    - CCACHE_HASHDIR=1
 
 anchors:
   envs: &common-job-env