Add python 3.4 to Travis CI build

[kjordahl]

Python 3.4 is now available on Travis.

[kjordahl]

Also, needed to fix the links to the NYC sample data, as the version changed on the website. It could be streamlined, the URL and filename are repeated many times. The source website is http://www.nyc.gov/html/dcp/html/bytes/districts_download_metadata.shtml#bcd.

[kjordahl]

Plot tests still failing on Travis: TclError: no display name and no $DISPLAY environment variable. Must have been some Travis environment change that led to using a different matplotlib backend, I guess.

[kjordahl]

The plotting tests are fixed. It is necessary to have the matplotlib.use('Agg') line before any other matplotlib imports.

Now tests are failing on one combination: Python 3.4 with pandas 0.12. It seems to be an issue with pandas' assert_series_equal that was fixed in later versions. Need to find a workaround.

[coveralls]

Coverage decreased (-4.09%) when pulling 0e42d9c on test/python34 into 761b5c6 on master.

[coveralls]

Coverage decreased (-4.09%) when pulling 9bd4f59 on test/python34 into 761b5c6 on master.

[kjordahl]

Dropped pandas 0.12.0 from the test matrix, added 0.14.0 release, and excluded the combination of Python 2.6 with pandas 0.14.0 that fails for no apparent reason, and finally have tests passing again here. One other problem is the postgis dependencies for ubuntugis are causing problems with dependencies for GDAL, so postgis tests are skipped on Travis for now.

Merging this for now, but test issues still have to be resolved.