Fix Travis build

Add "test" extra and install dependencies for said extra as detailed in:

https://stackoverflow.com/a/41398850/200603

.travis.yml

@@ -15,7 +15,7 @@ before_install:
 
 install:
   - travis_retry pip install -U pip wheel tox
-  - travis_retry pip install -U -r requirements.txt -e .
+  - travis_retry pip install -U -r requirements.txt -e ".[test]"
 
 script:
   - tox -e $TOX_ENV

setup.py

@@ -12,6 +12,14 @@ if sys.platform == 'darwin':
         print("Using lxml<2.4")
         lxml_requirement = "lxml<2.4"
 
+test_deps = [
+    # Test timeouts
+     "timeout_decorator",
+]
+extras = {
+    'test': test_deps,
+}
+
 setup(
     name="readability-lxml",
     version="0.7",
@@ -28,10 +36,8 @@ setup(
         lxml_requirement,
         "cssselect"
         ],
-    tests_require=[
+    tests_require=test_deps,
-        # Test timeouts
+    extras_require=extras,
-        "timeout_decorator",
-    ],
     classifiers=[
         "Environment :: Web Environment",
         "Intended Audience :: Developers",

tox.ini

@@ -16,5 +16,5 @@ deps=pytest
 # $PYTHONDIR\Scripts\pip.exe install *.whl
 sitepackages=True
 commands =
-    pip install -r requirements.txt
+    pip install -r requirements.txt -e ".[test]"
     py.test