Use 'python-dateutil' instead of 'dateutil' for requirement files
The former also is known to the 'pip install' command, and still works with conda install. Making requirements files compatible with 'pip install' is more relevant now that packages like numpy have a (binary) wheel available.

these are tailored to conda installs, so this will not work
 This _will_ work, because `conda` accepts `python-dateutil`.
 @chrish42 pls ping when this is green (I restarted the jobs).
 hmm, sees `python-dateutil` not defined in the default conda channel....?
 Works now with conda:

```
$ conda create -n test python-dateutil
Fetching package metadata: ....
Solving package specifications: .
Package plan for installation in environment /Users/chrish/anaconda/envs/test:

The following NEW packages will be INSTALLED:

    openssl:         1.0.1k-1
    pip:             6.1.1-py27_0
    python:          2.7.9-1
    python-dateutil: 2.4.2-py27_0
    readline:        6.2-2
    setuptools:      15.2-py27_0
    six:             1.9.0-py27_0
    sqlite:          3.8.4.1-1
    tk:              8.5.18-0
    zlib:            1.2.8-0

Proceed ([y]/n)?
```
 hmm, can you rebase on master and push again?
 Rebased, but two of the builds are still failing.
 @chrish42 so the older versions of dateutil fail here because they don't exist on conda.

They _can_ be built and posted to the `pandas` binstar channel. But would need to do for all arches. Alternative is to not change those for those builds in the ci file.
 Change for versions where python-dateutils is not available undone. Also rebased.
 @chrish42 thanks!
