Use a trick to speed up travis builds

Idea from https://blog.esciencecenter.nl/travis-caching-and-incremental-builds-6518b89ee889
and tool from https://github.com/MestreLion/git-tools/blob/master/git-restore-mtime

.travis.yml

@@ -36,6 +36,7 @@ before_install:
 - make travis-setup
 
 script:
+- curl -L https://raw.githubusercontent.com/MestreLion/git-tools/master/git-restore-mtime | python
 - agda --version
 - acknowledgements --version
 - make test-offline # disable to only build cache