Add conda-forge channel and remove libgfortran conda deps

[jchodera]

Fixes #119

[jchodera]

The current issue seems to be that pymc is being picked up from the anaconda default channel instead of the conda-forge channel:

The following NEW packages will be INSTALLED:
    assaytools-dev:  0.1.0-np19py27_0  omnia      
    cairo:           1.12.18-6                    
    fontconfig:      2.11.1-6                     
    freetype:        2.5.5-1                      
    icu:             58.1-1            conda-forge
    libgfortran:     3.0.0-1                      
    libiconv:        1.14-4            conda-forge
    libpng:          1.6.17-0                     
    libxml2:         2.9.4-4           conda-forge
    libxslt:         1.1.29-5          conda-forge
    lxml:            3.8.0-py27_0      conda-forge
    matplotlib:      1.4.3-np19py27_2             
    mkl:             11.3.3-0                     
    numpy:           1.9.3-py27_3                 
    pandas:          0.20.3-py27_1     conda-forge
    pint:            0.8.1-py27_0      conda-forge
    pixman:          0.32.6-0          conda-forge
    py2cairo:        1.10.0-py27_2                
    pymc:            2.3.4-np19py27_0   

The default channel pymc appears to be incorrectly built because libgfortran==1 is not provided as a dependency.

[jchodera]

Whoops, there's a typo:

Error: bad character '=' in package name dependency 'libxslt=1.1.29=2'

[jchodera]

Whoops: One problem was that we were still building python 3.4, which is no longer supported by conda-forge, so travis was mixing-and-matching conda-forge and default channels.

[jchodera]

Woohoo! This passes now.

@sonyahanson can you review and merge?

[sonyahanson]

Looks good!