chore: fix TravisCI build

Fixes #45
@aymericbeaumet Ha! I must say that I've copied this line as a monkey, without fully understanding it :-/ Shame on me. The thing is that without this line it wasn't working: 
https://travis-ci.org/karma-runner/karma-ng-html2js-preprocessor/jobs/37902879

Do you know what this line supposed to do / why it is here (Pawel looks up bash syntax feeling ashamed...).