travis: enable ccache
travis: enable ccache

Otherwise, running tests take an absurd amount of time.

kusma/fuselibs-public,nekronos/fuselibs-public,kusma/fuselibs-public,cbaggers/fuselibs-public,eksperts/fuselibs-public,Tapped/fuselibs-public,yupferris/fuselibs-public,Tapped/fuselibs-public,cbaggers/fuselibs-public,yupferris/fuselibs-public,Tapped/fuselibs-public,eksperts/fuselibs-public,fusetools/fuselibs-public,cbaggers/fuselibs-public,Tapped/fuselibs-public,fusetools/fuselibs-public,kusma/fuselibs-public,Tapped/fuselibs-public,yupferris/fuselibs-public,eksperts/fuselibs-public,kusma/fuselibs-public,fusetools/fuselibs-public,cbaggers/fuselibs-public,cbaggers/fuselibs-public,Tapped/fuselibs-public,kusma/fuselibs-public,yupferris/fuselibs-public,yupferris/fuselibs-public,eksperts/fuselibs-public,nekronos/fuselibs-public,nekronos/fuselibs-public,yupferris/fuselibs-public,nekronos/fuselibs-public,eksperts/fuselibs-public,yupferris/fuselibs-public,fusetools/fuselibs-public,fusetools/fuselibs-public,nekronos/fuselibs-public,eksperts/fuselibs-public,kusma/fuselibs-public,fusetools/fuselibs-public,eksperts/fuselibs-public,cbaggers/fuselibs-public,Tapped/fuselibs-public,kusma/fuselibs-public,cbaggers/fuselibs-public,nekronos/fuselibs-public,fusetools/fuselibs-public,nekronos/fuselibs-public