Bug 1317275 - Run pip check on Travis

Verifies installed packages have compatible dependencies, to help
prevent issues like bug 1324707. This will reduce the time taken to
review pyup bot PRs.

Example output if errors found:
```
Running pip check
celery 3.1.25 has requirement kombu<3.1,>=3.0.37, but you have kombu 4.1.0.
```

.travis.yml

@@ -26,6 +26,7 @@ matrix:
       install:
         - pip install --disable-pip-version-check --require-hashes -r requirements/common.txt -r requirements/dev.txt
       script:
+        - pip check
         - python lints/queuelint.py
         - flake8 --show-source
         - isort --check-only --diff --quiet

runtests.sh

@@ -1,5 +1,8 @@
 #!/bin/sh
 
+echo "Running pip check"
+pip check || { exit 1; }
+
 echo "Checking CELERY_QUEUES matches Procfile"
 ./lints/queuelint.py || { exit 1; }