Use ccache for the unified builds

This may speed up our builds considerably
Reviewed-by: NEmilia Käsper <emilia@openssl.org>

.travis.yml

 language: c
+cache: ccache
 
 addons:
     apt:
         packages:
+            - ccache
             - clang-3.6
             - gcc-5
             - binutils-mingw-w64
@@ -62,6 +64,7 @@ before_script:
           srcdir=../_srcdir;
           mkdir _build;
           cd _build;
+          CC="ccache $CC";
       else
           srcdir=.;
           cd _srcdist;