Well, since you cannot cancel a build which is already waiting in a queue, you have to think about it before committing :-)
Travis-Ci a has nice feature which allows to skip making a build for certain push. Just add
[ci skip] into your commit's message to let Travis-Ci know, that you do not want to perform tests for the next push.
git commit -m 'documentation update [ci skip]'
It is very useful when you are working things unrelated to application's code, ie. documentation, styling, etc.