set ujson as optional dependency, test CPython builds with ujson

.travis.yml

@@ -10,8 +10,9 @@ install:
   - pip install coveralls
   - pip install -r requirements.txt
   - pip install -r requirements-dev.txt
+  - if [[ $TRAVIS_PYTHON_VERSION != 'pypy'* ]]; then pip install ujson; fi
 script:
   - nosetests -v --with-flaky --no-flaky-report --with-coverage --cover-package=telegram/
-  - 'if [ $TRAVIS_PYTHON_VERSION != 3.3 ] && [ $TRAVIS_PYTHON_VERSION != pypy3 ]; then pre-commit run --all-files; fi'
+  - if [ $TRAVIS_PYTHON_VERSION != 3.3 ] && [ $TRAVIS_PYTHON_VERSION != pypy3 ]; then pre-commit run --all-files; fi
 after_success:
   coveralls

setup.py

@@ -34,6 +34,9 @@ with codecs.open('README.rst', 'r', 'utf-8') as fd:
           long_description=fd.read(),
           packages=find_packages(exclude=['tests*']),
           install_requires=requirements(),
+          extras_require={
+              'json': 'ujson',
+          },
           include_package_data=True,
           classifiers=[
               'Development Status :: 5 - Production/Stable',