Opencellid

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New
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- Add support for Python 3.6, 3.7. [Ash]

Closes 11

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Changes
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- Updated CodeClimate settings. [Ash Wilson]

Closes 3
- Updated download URL for OpenCellID feed. [Ash Wilson]

Updated tests to avoid Travis-CI silence timeout.

Closes 2

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Changes
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- Improved error handling for bad API token and rate limiting
situations. [Ash Wilson]

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Changes
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- Support update from UnwiredLabs or Mozilla Location Services. API
changed, now use update_feed() method and source will be chosen
depending on the presence of an API key. [Ash Wilson]

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New
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- Unit and integration tests. [Ash Wilson]
- Creates an OpenCellIdFeed object which can be updated from web and
iterated over. [Ash Wilson]