travis: split up into 3 builds

This commit distributes the CI tests into 3 independend builds, by
splitting the integration test run and unit test coverage.

To better handle the extra cases, we define a build matrix with the
three build types (RACE and LINT, ITEST, COVER).

.travis.yml

@@ -1,10 +1,12 @@
 language: go
+go:
+  - "1.11.x"
+
+env:
   matrix:
-  include:
+    - RACE=true LINT=true
-  - go: "1.11.x"
+    - ITEST=true
-    env: RACE=true LINT=true
+    - COVER=true
-  - go: "1.11.x"
-    env: ITEST=true COVER=true LINT=true
 
 sudo: required
 install: