Check if a V2 deployment is present. If not, proceed with V3. Fixes #548

[gglucass]

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• Your code builds clean without any errors or warnings
• You are using approved title ("feat/", "fix/", "docs/", "refactor/")

A description of the changes proposed in the pull request:
Fixes #548

Adds robustness in the case a Uniswap V2 deployment is not present on a chain.

Tests performed by the developer:
Swapping on Celo didn't work at first, now it does.

Tips for QA testing:
Swap USDGLO for USDC on Celo using current version which should fail, and this version which should succeed.

[fengtality]

@gglucass See comment in issue #548

[gglucass]

@fengtality - responded in #548 . Note that this PR is to prevent uniswap/router swaps from failing in the event the chain has no UniswapV2 deployment, which is the case for Celo.