travis: Move linter to a separate job.

.travis.yml

@@ -4,12 +4,14 @@ python:
   - "3.4"
   - "3.5"
   - "3.6"
+matrix:
+  include:
+  - python: "3.6"
+    env: TEST_SUITE=linter
 install:
   - pip install pycodestyle==2.3.1
   - pip install ./zulip
   - pip install ./zulip_bots
   - pip install ./zulip_botserver
 script:
-  - tools/lint
+  - tools/travis_tests
-  - python -m unittest discover -v zulip_botserver
-  - tools/test-bots

tools/travis_tests

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+
+set -ev
+
+CURRENT_DIR=$(dirname "$0")
+cd "$CURRENT_DIR"
+cd ..
+
+if [ "$TEST_SUITE" == "linter" ]
+then
+    tools/lint
+else
+    python -m unittest discover -v zulip_botserver
+    tools/test-bots
+fi