use a different backend for mpl viz on travis
use a different backend for mpl viz on travis

avmarchenko/exatomic,exa-analytics/exatomic,exa-analytics/exatomic,exa-analytics/atomic,tjduigna/exatomic,tjduigna/exatomic,avmarchenko/exatomic,exa-analytics/exatomic,alexvmarch/atomic,exa-analytics/exatomic,alexvmarch/atomic,avmarchenko/exatomic,exa-analytics/atomic,tjduigna/exatomic