Add cron job for Travis

[hsiaoyi0504]

As title, I think that will make us find the problem faster.
As I remember, it can be setup like running every day.
@greglandrum Can you setup this for us ?

[greglandrum]

Travis runs after ever commit, so there's no need to run it every day.

[hsiaoyi0504]

@greglandrum I agree with you that basically, Travis runs after every commit. However, in this repo case, the installation not only depends on the commit of this repo itself, but also depends on upstream rdkit repo (HEAD version) and the version of libraries installed installed by homebrew (such as boost-python). Every commit in homebrew-core repo and commit in upstream rdkit repo will not start running a Travis travis of this repo. That's the reason why I think it's a need to run it every day through cron job https://docs.travis-ci.com/user/cron-jobs/.

[hsiaoyi0504]

@greglandrum

[greglandrum]

ok, I set up the cron job to run daily.
let's see if it helps anything

[hsiaoyi0504]

Thanks !