Update installation requirements

I needed these but accidentally forgot to add them into the
documentation.

README.md

@@ -15,11 +15,11 @@ location.
 
 ## Requirements
 
-This program requires Python3, `pandas`, and `tqdm`.
+This program requires Python3, `python-dateutil`, `pandas`, and `tqdm`.
 These can be installed via:
 
 ```
-pip3 install pandas tqdm
+pip3 install python-dateutil pandas tqdm
 ```
 
 Or, if you use `pipenv`, you can use the following to install from the

chimet-scraper.py

@@ -9,7 +9,7 @@ try:
     import tqdm # cool progress bar
 except ImportError as e:
     raise ImportError("Could not find needed modules. Please run \n"
-        "pip3 install python-dateutils pandas tqdm\n"
+        "pip3 install python-dateutil pandas tqdm\n"
         "to install all modules. Original error was {e}") from e
 
 def parse_date(date_str):