Makes prawcore dep explicit

requirements.txt

@@ -1,6 +1,7 @@
 arrow==0.9.0
 decorator==4.0.10
 praw>=4.0
+prawcore
 PyYAML==3.12
 requests==2.12.1
 six==1.10.0

setup.py

@@ -24,7 +24,7 @@ setup(
                  "Intended Audience :: End Users/Desktop",
                  "Programming Language :: Python"],
     packages=["shreddit"],
-    install_requires=["arrow", "backports-abc", "praw>=4", "PyYAML", "requests", "six", "tornado"],
+    install_requires=["arrow", "backports-abc", "praw>=4", "prawcore", "PyYAML", "requests", "six", "tornado"],
     package_data={"shreddit": ["shreddit.yml.example"]},
     entry_points={
         "console_scripts": [