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.

.travis.yml

@@ -15,7 +15,7 @@ env:
   global:
     - GOCACHE=$HOME/.go-build
   matrix:
-    - RACE=true LINT=true
+    - RACE=true
     - ITEST=true
     - COVER=true 
 
@@ -32,19 +32,14 @@ script:
   # Common for all builds.
   - make btcd
 
-  # Run linter if LINT=true.
-  - 'if [ "$LINT" = true ]; then make lint ; fi'
-
   # Run unit tests with race condition detector.
   - 'if [ "$RACE" = true ]; then make unit-race ; fi'
 
   # Run integration tests.
-  - 'if [ "$ITEST" = true ]; then make build ; fi'
-  - 'if [ "$ITEST" = true ]; then make itest-only ; fi'
+  - 'if [ "$ITEST" = true ]; then make build && make itest-only; fi'
 
   # Run unit tests and generate coverage report.
-  - 'if [ "$COVER" = true ]; then make unit-cover; fi'
-  - 'if [ "$COVER" = true ]; then make goveralls; fi'
+  - 'if [ "$COVER" = true ]; then make lint && make unit-cover && make goveralls; fi'
 
 after_script:
   - echo "Uploading to termbin.com..." && find *.log | xargs -I{} sh -c "cat {} | nc termbin.com 9999 | xargs -r0 printf '{} uploaded to %s'"