Drop Python 3.4 support

[hayfield]

pyIATI should drop support for Python 3.4.

3.4 support should be dropped because:

• Current versions of operating systems come with 3.5 or 3.6 - these should support everything that 3.4 does
• 3.7 will be out soon - supporting an ever-growing list of minor versions without dropping any is a bad thing
• It's going EoL in 12 months time
• It adds very little benefit, while increasing the time tests take to run, and occasionally causing problems
• 3.4 lacks a number of features that could allow code to be simplified or made more robust (for example, around the testing of decorators).
• The reason for supporting it initially was that early builds of pyIATI happened to work against it (but not 3.3, hence that was not supported)

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• Remove from Travis
• Remove from setup.py
• Remove from other places it's mentioned
• Do a global search for 3.4 and remove / update code as appropriate to remove custom workarounds