travis.yml: add DB=sqlite to env matrix.
travis.yml: add DB=sqlite to env matrix.

danstaines/ensembl,at7/ensembl,james-monkeyshines/ensembl,mjg17/ensembl,muffato/ensembl,danstaines/ensembl,james-monkeyshines/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,Ensembl/ensembl,muffato/ensembl,mjg17/ensembl,muffato/ensembl,at7/ensembl,mjg17/ensembl,at7/ensembl,danstaines/ensembl,james-monkeyshines/ensembl,Ensembl/ensembl,mjg17/ensembl,muffato/ensembl,Ensembl/ensembl,at7/ensembl