updated aiohttp version to be inline with discord.py

requirements.txt

@@ -1,5 +1,5 @@
 git+https://github.com/Rapptz/discord.py@rewrite#egg=discord.py[voice]
-aiohttp==3.0.1
+aiohttp<2.3.0,>=2.0.0
 beautifulsoup4==4.6.0
 pillow==5.1.0
 numpy==1.14.3