Add msgpack to extras_requires

requires/development.txt

@@ -1,4 +1,5 @@
 -r testing.txt
 -r installation.txt
+-e .[msgpack]
 sphinx>=1.2,<2
 sphinxcontrib-httpdomain>=1.3,<2

requires/testing.txt

@@ -1,4 +1,3 @@
 coverage>=3.7,<3.99  # prevent installing 4.0b on ALL pip versions
 mock>=1.3,<2
-u-msgpack-python>=2,<3
 nose>=1.3,<2

setup.py

@@ -54,6 +54,9 @@ setuptools.setup(
     packages=setuptools.find_packages(),
     install_requires=install_requires,
     tests_require=tests_require,
+    extras_require={
+        'msgpack': ['u-msgpack-python>=2.5.0,<3']
+    },
     namespace_packages=['sprockets', 'sprockets.mixins'],
     test_suite='nose.collector',
     python_requires='>=3.5',