CI: Use ccache to speed up the build

Speeds up recompilation by caching previous compilations and detecting
when the same compilation is being done again. [skip appveyor]

.travis.yml

@@ -1,5 +1,8 @@
 language: cpp
 
+cache:
+  ccache: true
+
 env:
   global:
   # AWS S3 creds
@@ -8,6 +11,12 @@ env:
   # secret
   - secure: "JRQVU2zgC3hY6CEY+Crmh/upp93En0BzKaLcsuBT538johNlK7m5hn3m2UOw63seLvBvVaKKWUDj9N986a3DwcXxWPMyF/9ctXgNWy39WzaVWxrbVR5nQB1fdiRp5YEgkoVN+gEm3OVF7sV5AGzh5/8CvEdRCoTLIGgMGHxW9mc="
 
+  # ccache
+  - USE_CCACHE=1
+  - CCACHE_COMPRESS=1
+  - CCACHE_MAXSIZE=200M
+  - CCACHE_CPP2=1
+
 matrix:
   include:
     - os: osx