Run eslint via tox.

This makes the .travis.yml simpler again and fixes the fact that the exit
status of a command was ignored because of the "|| true".

.travis.yml

@@ -36,8 +36,7 @@ install:
   - python scripts/dev/ci_install.py
 
 script:
-  - '[[ $TESTENV != eslint ]] && tox -e $TESTENV -- -p no:sugar tests || true'
+  - tox -e $TESTENV -- -p no:sugar tests
-  - '[[ $TESTENV == eslint ]] && eslint qutebrowser || true'
 
 matrix:
   exclude:

tox.ini

@@ -194,3 +194,9 @@ deps =
 commands =
     {envpython} scripts/link_pyqt.py --tox {envdir}
     {envpython} scripts/dev/freeze.py {posargs}
+
+[testenv:eslint]
+skip_install = True
+deps =
+whitelist_externals = eslint
+commands = eslint qutebrowser