Use Travis to block PRs with large files

[sappelhoff]

fixes #156

I am trying to ask Travis to check for file sizes now, blocking everything > 500kb.

[choldgraf]

can we test if this works by committing a big file to this PR once and seeing if travis fails? we can use git commit --amend --no-edit so it doesn't get committed to the git history

[sappelhoff]

I added a ./bigfile of 1MB size, and Travis caught it, see the log: https://travis-ci.org/bids-standard/bids-examples/builds/528367294?utm_source=github_status&utm_medium=notification

I'll revert that commit now, then git amend, and push -f

[choldgraf]

sweet! I'll give this a merge and hopefully it'll work moving forward! Thanks for the addition @sappelhoff