Drop Python 3.4 support

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)

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

IATI / pyIATI