Replace Service Context, Removed dotenv-python from requirements, Changed chromadb version to 0.5.3

[speedwagon1299]

Fix for Issue #77

The usage of llama_index.core.ServiceContext is now deprecated in LlamaIndex. The official documentation recommends switching to llama_index.core.Settings

Two more issues pertaining to the requirements.txt affecting other notebooks:

1. dotenv-python overwriting python-dotenv in pip resulting in the following error:

ImportError: cannot import name 'load_dotenv' from 'dotenv' (unknown location)

Solution: Remove dotenv-python from requirements.txt

2. chromadb==0.5.15 causing vectorstore to crash python kernel during its invoking in reliable-rag.ipynb
   and possibly others importing from Chroma.

Solution: Change version to chromadb==0.5.3 in requirements.txt

I have added these two changes part of this PR since I believe they are too minute to warrant their own PR. If I should remove these changes and only fix the aforementioned issue, I am ready to revert to only the changes specified in the issue. Please let me know if the following changes work. Thank you