In NEST, min_delay is no longer set in the defaults of each synapse but min_delay is set globally in the NEST kernel

Change the NEST-version that Travis downloads to current master branch
Change script to download and re-build NEST every time so that Travis always checks the current master

…on of cmake

 into mschmidt87-fix_min_delay_error

# Conflicts:
#	.travis.yml
#	ci/install_nest.sh

… of NEST models.

…es for the start time of DCSource or for the simulation time)

…for `pyNN.neuron`, using NMODL description from Christian Roessert

… rather than 0 mV, during the refractory period (to match the behaviour of other IF models).

…, so it is not necessary to use the MyModule approach.

Also modified the NEURON implementation to increment the eta and gamma variables at spike onset rather than after the refractory period. This matches the description in Pozzorini et al., and the behaviour of the NEST implementation.

…#114 has been fixed.

Generalized integrate-and-fire model

# Conflicts:
#	pyNN/neuron/standardmodels/cells.py

# Conflicts:
#	pyNN/nest/cells.py
#	pyNN/neuron/standardmodels/cells.py