Add repeat-distance to text symbolizer properties.

Update collision detector to handle minimum-distance and repeat-distance seperately.
Update placement_finder to use repeat-distance.
Update group symbolizer to handle minimum-distance and repeat-distance.
Add additional test cases to demonstrate use of new attribute repeat-distance by itself and combined with minimum-distance.
Merge branch 'master' of github.com:mapnik/mapnik into jh-min-repeat-distance
Merge branch 'master' of github.com:mapnik/mapnik into jh-min-repeat-distance
fix compile with gcc
Merge branch 'mapycz-3.x-fix-compile'
Merge branch 'jh-min-repeat-distance' of git://github.com/MapQuest/mapnik into MapQuest-jh-min-repeat-distance

Conflicts:
	include/mapnik/renderer_common/process_group_symbolizer.hpp
	src/text/text_properties.cpp
Merge branch 'MapQuest-jh-min-repeat-distance'
Travis: drop concurrent jobs for clang
optimize set_symbolizer_property_impl
remove unneeded code
fix return type
fix simply option + tests for #1484 and #1992
load_map: always parse transform so that xml warning keep working
pixel snapping for shields (and points) - closes #1866/refs #1316/amends 2fcd53134
update expected image after pixel snapping change
Travis: remove lto testing code
attempt to repair postgres install
travis: alt gdal approach
travis: another try at gdal