setup.py: Add 'cachelib' dependency (#352)

Since #308 added a hard requirement for `cachelib`, add it
to user-facing `setup.py` as well (it was previously
only added to dev-dependencies).

Author: ix5

setup.py

@@ -3,7 +3,7 @@
 # Metadata goes in setup.cfg. These are here for GitHub's dependency graph.
 setup(
     name="Flask-Caching",
-    install_requires=["Flask"],
+    install_requires=["cachelib", "Flask"],
     tests_require=[
         "pytest",
         "pytest-xprocess",