Updating to dev-master is the worst thing you can do right now. This would install the current branch all changes for the upcoming release flow into. Installing the dev-master extension will most likely create white pages on your forum and in the worst case will break your forum permanently because if database changes are applied there's no way to restore them.
Aka, don't do this.
In this situation I would go against krombox advice and edit core files. Once you upgrade to beta 9, composer will tell you "there are edited files in flarum/core, overwrite?" and you answer with yes.
I'm glad composer is against installing dev-master right now, these are compliant with the upcoming beta 8, not beta 7.1.
