Corrects aiohttp error discord-py throws. (#54)

Error when using latest aiohttp:
```Successfully built aiohttp asyncpg discord.py humanize uvloop ciso8601 pycares multidict yarl idna-ssl regex tzlocal websockets numpy cffi pycparser
discord-py 1.0.0a0 has requirement aiohttp<3.5.0,>=3.3.0, but you'll have aiohttp 3.5.4 which is incompatible.
```

Correction is to set version used to 3.5.0.

src/requirements.txt

@@ -1,5 +1,5 @@
 aiodns
-aiohttp
+aiohttp==3.3.0
 asyncpg
 dateparser
 https://github.com/Rapptz/discord.py/archive/860d6a9ace8248dfeec18b8b159e7b757d9f56bb.zip#egg=discord.py
@@ -7,4 +7,4 @@ humanize
 uvloop; sys.platform != 'win32' and sys.platform != 'cygwin' and sys.platform != 'cli'
 ciso8601
 pytz
-timezonefinder
+timezonefinder