travis: collapse commands into single instance to ensure exit on fail
travis: collapse commands into single instance to ensure exit on fail

Before this commit, if for example the linter failed, then we would go
on to all the other tests rather than halting. We fix this by instead
chaining the relevant commands, and eliminating the LINT env variable
in the build matrix.

LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd